Nick Bitel, the marathon's chief executive, said last night that Britain's efforts to stage top international sporting events were being undermined by the lack of legislation against "ambush marketing".
Several international sports companies, including the American sportswear company Nike, are believed to be planning unofficial campaigns to capitalise on this year's marathon, which attracts a television audience in the United Kingdom of 6 million.
Nike yesterday unveiled a new poster in a London-wide campaign aimed to coincide with the marathon. The company said it was "not trying to hijack" the event.
"We are not an official sponsor but around the London Marathon people are very interested in running," its spokes-man, Graham Anderson, said. "It is a high-profile event and all companies with an interest in running will be down there."
Ambush marketing is not illegal. Last year Nike bought up every bus-stop poster site around the course for unofficial advertisements featuring a disabled competitor. It was the latest in a succession of "ambushes" by Nike, a great rival of Asics, the official sponsor of the event.
In a previous year Nike hired a huge advertising billboard at a key site near the Cutty Sark tea clipper in Greenwich, which race organisers then had to obscure from television cameras.
On another occasion, the sports company sought planning permission for a giant inflatable running shoe to be attached to a building overlooking a prime site. Race organisers managed to block the application.
This year, a team of race officials will spend the night before the race patrolling the perimeter and clearing unofficial advertising. The final sweep will be carried out just 20 minutes ahead of the runners. Mr Bitel said: "The marathon is a difficult event to protect because we are not in a clean stadium. It is much easier to ambush an open-air event."
In the next three years, more than 20 world and European sporting championships are due to be staged and the Sports Council recently announced a pounds 3m lottery- funded kitty to help sports governing bodies bring events to Britain.
Mr Bitel, a London lawyer, said: "Sponsorship is absolutely pivotal to your ability to stage events. If you cannot keep your advertisers happy because of the ambushing antics of another company then it will cost you on the bottom line."
He said many major sporting events, such as the Euro 96 football championships, were run on tight budgets and made little direct profit.
Mr Bitel said that ambush marketing was a particular problem in Britain because laws on the subject were weaker than in any comparable country. Victims of ambush marketing in Britain have little legal recourse other than through legislation designed to prevent the "passing off" of one product as another.
The Football Association recently failed to prevent Trebor Bassett, the sweet company, from using England shirts in their promotions during Euro 96. The FA has an arrangement with Snickers, made by Mars.
Mr Bitel would like to see American-style legislation introduced to give event organisers greater protection against ambushing. "They are much stricter in their protection of intellectual property," he said.
Nike's saturation advertising during Euro 96 convinced 23 per cent of the population that they were the official sponsor, when in fact it was Umbro. Some people have been led to think that Britvic are associated with Manchester United after a poster campaign for the Red Card sports drink featuring an Eric Cantona lookalike in a red football shirt with the collar turned up.
American Express has also run several campaigns around the Olympics, which is sponsored by its rival Visa.
Mr Bitel predicted that there could be difficulties in attracting sponsors for the 1999 Rugby World Cup in Britain after Guinness saturated the last Steinlager-sponsored event.
Some companies feel that they get better rewards from sponsoring individuals rather than sporting events.