Alastair Campbell has urged employers to take a leaf out of Tony Blair's book and show more understanding towards people suffering from mental illness.

Speaking exclusively to The Independent on Sunday, Mr Campbell revealed how depression gave him the strength needed to work for the former prime minister and helped him cope with the demands of his job as communications chief.

Mr Campbell is plagued by recurrent bouts of depression, which he has learnt to accept as part of an otherwise happy life.

His frank account comes on the eve of Depression Awareness Week, which this year seeks to highlight the stigma millions continue to face in the workplace. Supportive bosses and colleagues could help the majority of sufferers keep working, say campaigners. According to Mr Campbell, dealing with a breakdown gave him the strength to cope with anything No 10 threw his way. He said: "I definitely feel I was in many ways a stronger person after I had recovered. I do not think I would have been able to do the job I did for Tony Blair without having had that strength.

"So in some ways I look back on the breakdown as a very positive experience because I sorted myself out and I have led, in many ways, a charmed life personally and professionally."

Mr Campbell spoke candidly to Mr Blair about his breakdown and had his full support from the start. But new research, which will be published tomorrow by Depression Alliance, found that 75 per cent of sufferers are too scared to talk openly about their mental illness at work.

Nearly a third of respondents believed they had been turned down for a job or promotion, while a similar number had experienced bullying, because of their depression. These startling figures come amid growing evidence that demonstrates the importance of work for mental wellbeing. But despite recent government efforts to help mentally ill people into work, stigma remains rife.

Emer O'Neill, chief executive of Depression Alliance, said: "In the 13th Depression Awareness Week we want to raise awareness about how treatable depression is and open up these discussions in the workplace because working can be an integral part of getting better.

"Employers need to realise that 80 per cent of people with depression will make a full recovery if they get the right treatment and support at the right time."

One in four of the UK population will be affected by depression at some point. Mental health problems cost employers £26bn every year, but for sufferers the costs of not working can be devastating.

The snooker star Graeme Dott, 30, plays his opening game of the world championships on Tuesday. Last week the world number two revealed that he was suffering from clinical depression which left him unable to play snooker or face the world for months.

Mr Dott said: "I knew eight months ago that something was wrong but I had no idea it was depression. It has been virtually impossible to play snooker as I had no energy and I just did not want to be there. I even felt paranoid that people could tell there was something wrong with me, so I stopped going out."

Three months after he sought help Mr Dott has started counselling and anti-depressants, and with the support from his fellow-player Ronnie O'Sullivan, who has also suffered from depression, he is starting to recover.

He said: "I knew if I did not play this week I would feel worse, and to be honest, I do not want to give into the depression. I do not know how I am going to cope on Tuesday but depression has taken snooker away from me and I have to get it back."

Mr Campbell's first novel is to be published in November, two years after he first revealed the depths of his despair in The Independent on Sunday.

All in the Mind has a psychiatrist as the main character but Mr Campbell denies the book is autobiographical. He said: "It is about a lot more than depression or alcoholism. At its heart it is about the relationships in a psychiatrist's life and the pressures people bring to bear upon each other."