Tottenham Hotspur used it. Monica Seles wouldn't be playing today without it. Even England rugby players believe in it . . .

Sports psychology is as much the key to success today as any training or coaching.

Monica Seles, one of the most confident and resilient tennis competitors imaginable, became almost reclusive after she was stabbed in the back by a spectator during a changeover in 1993.

Among those Seles thanked for saving her career 27 months later was Jerry May, a clinical psychologist, who helped her overcome the trauma and nourished her desire to return to the court. She has since won the Canadian Open and finished runner-up to Steffi Graf after a magnificent United States Open final.

"It's still hard," Seles said, "and there are days when you struggle with it, but I have someone I can talk to and I go through exercises that help me to sit down in that chair again."

Seles is an exceptional case, but her experience is an example of a phenomenal development in sport. Success nowadays is engineered not only on the training grounds, in the gym, and sometimes in a laboratory, but also in the mind. Mental preparation is considered important enough to have become a growth industry.

Although there is a tendency to regard sports psychologists with scepticism and suspicion, every coach worthy of the name understands that a fusion of innate skill and intellect can work wonders.

May is based at the University of Nevada School of Medicine, and his reputation as a sports consultant was built on a lengthy association with the United States Olympic ski team, who adopted his relaxation techniques. According to May, "you can teach people to relax very profoundly."

American sports pioneered research into the power of the mind in relation to athletic achievement, innovating techniques such as visualisation. This was used by Dwight Stones, the high-jumper, who reckoned that during his mental preparation he could picture himself soaring over the bar as clearly as the moment when he actually performed the feat.

Once considered exclusive to America and eastern Europe, a feature of Cold War medalmania, the use of sports psychology has proliferated world wide. Australia quickly caught on, employing psychologists at its Institute of Sport.

Those responsible for implementing John Major's policy of "Raising The Game" and establishing a British Academy of Sport realise they cannot afford to underestimate the part psychology plays in the arenas of the world.

The British Association of Sport and Exercise Sciences (BASES), formed 12 years ago, has a membership of 496. Of these, 50 are accredited sport and exercise psychologists, many of whom work with national and Olympic squads.

BASES is part of the National Coaching Federation, grant aided by the Sports Council, whose Sports Science Education Programme reports that the intake of sports science degree students at colleges and universities has quadrupled.

While BASES covers sports from archery to trampolining, the Olympic sports appear more inclined to avail themselves than a number of the professional groups, notably football.

John Syer, a consultant to Tottenham Hotspur in the early 1980s, has worked with a number of professional sports since. "No football team has asked me in the last couple of years to work with them," he says, "although a lot of managers probably know of our company. I think more people work in cricket than football."

Other high-profile sports have opened their minds. Dr Austin Swain, of Loughborough University, converted England's rugby union team, working with the individual players. "Principally," Tony Underwood says, "he has helped me calm my nerves before a game with relaxation techniques. And during a game there is positive thinking, so that if you do make a mistake, however trivial, it doesn't play on your mind; you're going to be able to shut it away and get on with the game."

Syer, 58, is one of the country's most experienced practitioners, an example of how sports psychology has expanded. In 1979, he formed a consultancy, Sporting Bodymind, in association with Christopher Connolly, an American water-skier. The company now has a staff of six and also works with teams of businessmen.

"I think we in Britain are catching up," Syer said. "But there was a lot of catching up to do, because when we started the Americans were way ahead, and, of course, the east Europeans, too. I think the east Europeans were very science-orientated. It's often been confused with the medical profession. I don't think it belongs in a bracket with the medical profession. When it works best it's not diagnostic, it's more educational."

Syer's sport was volleyball. He represented Britain and for 10 years was Scotland's national coach. His CV as a sports psychologist includes Glamorgan and Somerset county cricket clubs and the Cleveland Browns, in the NFL. For the past 10 years he has worked with the British Cycling Federation, Chris Boardman numbering among those he has advised.

"There have been big changes in the last five years or so," Syer said. "When Sue Campbell was with the National Coaching Foundation, she gave it a boost, as did Kevin Hickey when he became secretary of the British Olympic Association.

"The only thing is that it's still very difficult, if a coach wants a sports psychologist, to know what he or she is going to get. It has to be done by interview, because there are different brands of psychology, and some are much more analytical than others. I focus on performance and have nothing to do with analysing people. I help people discover what they can do rather than what they should be doing."

But is stimulating the mind to improve performance totally ethical, when so many athletes have been banned for using drugs to enhance performance?

Underwood has no qualms. "The simple answer I would have to that," he says, "is that whereas with drugs you are taking a substance in addition to what you have in your body, using someone to increase and enhance your mind power is the same as doing body building to increase and enhance your muscles. Using the power of your mind is a physical prowess of your own - a neglected one in the past - and obviously something that can be tapped into and exploited and used."

Syer concurs. "I don't think it's giving an unfair advantage, because it's the athlete using his or her own mind to improve their performance. It's work, an extra form of training. And in terms of relationships within a team, it becomes much more complex. That's a skill that we all learn in our different fields, starting with the family."

Tottenham were his first major clients, from 1980 to 1985. During this period Spurs won the FA Cup in 1981 and 1982 and the Uefa Cup in 1984, but the psychologist's arrival was treated with a degree of scepticism.

"White coats at White Hart Lane was the Daily Mirror headline," Syer recalled, "and another paper had a big black couch on the page. I completely understand that, because the layman doesn't really know the difference between psychiatry, psychology and sports psychology."

Job definition was a problem when Keith Burkinshaw, the Tottenham manager, was preparing to introduce Syer and his partner, Connolly, to the players.

"What should I call you?" Burkinshaw asked.

"Well, we're sports psychologists," Syer said.

"I can't call you psychologists," Burkinshaw told him.

"Well...mental training."

"We can't call you mental, can we?"

Burkinshaw resolved the situation, telling the players: "This is John and Chris and they're going to be with us for a while."

Natural psychologists, or motivators - characters of a distinctive personality, able to motivate teams, either as fellow players or supervisors - have been a part of sport since organised games became a feature of life in the Victorian era.

Prominent among the naturals are football managers, coaches and trainers, the majority of whom would consider the state of players' minds before matches to come within their sphere of influence.

Bill Shankly, of Liverpool, was a classic example, keen to extend his influence to undermine the opposing team if possible. Kevin Keegan remembers the time when a plaque - "This is Anfield" - was placed above the tunnel leading from the dressing-rooms to the pitch.

Keegan said: "Shanks was talking to Joe Harvey, at the top of the steps leading to the tunnel, about an hour before a game against Newcastle. I was just passing by with some tickets when Malcolm Macdonald saw the plaque and said jokingly: 'I told you we had found the right place, Boss'. 'Wait till you get out there, son, and you'll know you're at Anfield,' said Shanks. 'You can run, but you canna' hide!' "

Syer acknowledges the traditional mystique of football lore, from the trainer's "magic sponge" to the wisdom of boot-room philosophers. "Ritual is a way to evoke feeling that an athlete might need," he said. "But of course if you don't examine the rituals they might be out of date and doing more harm than good.

"It has to be quite clear, if I'm working with a coach or with a professional team, or whatever, I am an assistant to the coach, as is the physical trainer. There is a grey area. A sports psychologist has to be very careful, first of all never to make any comment about technical skill or technical ideas or tactics. That would really be intruding."