David Shayler served six months for breaking the Official Secrets Act

Mr Shayler, who was jailed for breaking the Official Secrets Act, said he would be campaigning against the Iraq war.

He said he was "neither left or right", but was concerned about what he called Labour's attack on democratic rights.

Mr Shayler, who is from Middlesbrough, said he plans a "serious" campaign but accepted he was unlikely to overturn Mr Blair's 17,713 majority.

If things go right, the people of Britain might again be dancing in the streets over another 'Portillo moment' David Shayler

The former counter-terrorism officer said: "Our democracy is in crisis and unless we act now, there will be no democracy left fighting for.

"If we can encourage a large protest vote in Sedgefield, it would send a clear message to the politicians of this country that the people have had enough of autocratic leaders who fail to represent their constituents."

He said that "if things go right, the people of Britain might again be dancing in the streets over another 'Portillo moment'" - a reference to Tory Michael Portillo's defeat at the 1997 general election, one of the biggest political upsets of recent years.

He claimed what he called Mr Blair's misguided actions had put the country at greater risk of dying in a terrorist attack.

Motivation

Mr Shayler said he had planned to stand against the prime minister at the last election but had been tied up by his court case at the time.

The ex-MI5 agent was sentenced to six months in jail in November 2002, following his conviction for breaking the Official Secrets Act.

He revealed secret documents to the Mail on Sunday newspaper in 1997, arguing he had a public duty to expose malpractice within the security services.

The judge in the case accepted Mr Shayler had been motivated by a desire to expose wrongdoing, not by the money he received for the article.

But the leaks had potentially placed the lives of secret service agents at risks, he added.