Mr Campbell was Tony Blair's spokesman until 2003

The former No 10 media chief said 60% of bosses say they would not employ someone with a mental health problem.

In an interview to mark World Mental Health Day, he said the media was reluctant to discuss the issue.

He said he still suffered bouts of depression, including while at No 10, but said he had found ways to cope.

It was while he was news editor of the Sunday edition of the newly-launched Today paper in 1986, that Mr Campbell suffered a nervous breakdown.

You sort of learn to how to deal with it, but some people don't Alastair Campbell

He told BBC Breakfast that since then he had suffered periodically from depression, without really knowing what caused it.

"You sort of learn to how to deal with it, but some people don't, some people get it far, far worse than I do," he said.

He said he had been supported by his partner, his family and by Mr Blair, who employed him in 1994.

"I had somebody who was then leader of the Opposition, going to be prime minister, who knew about the background that I had but felt that it wasn't a problem," he said.

'It felt terrible'

"I felt that was quite a positive sort of signal to send out, because six out of ten employers have said they wouldn't consider taking on someone with a mental health problem."

Mr Campbell, who will make a speech at the Mental Health Media Awards later, said the media was reluctant to talk about mental health issues.

He said the lowest point in his time at Downing Street came when government scientist Dr David Kelly committed suicide after he was named as the possible source of a BBC story on the government's Iraq dossier.

Asked how he felt about the affair, Mr Campbell said: "The honest, straightforward answer is it all felt terrible, the whole time.

"I don't think that was depression, that was just the fact you were dealing with what was, without any shadow of a doubt, for me the worst period for my time in Downing Street - and obviously for somebody like him, who I never met but who felt so badly about what was going on that he felt compelled to take his own life."