Stephen Byers has quit the government after months of speculation about his future.

At a hastily-convened Downing Street press conference, the cabinet minister announced he was resigning as transport secretary.

His decision was widely seen as an attempt to pre-empt his inevitable sacking in a summer reshuffle.

Downing Street insist the minister made his own decision to go - although Tony Blair did not attempt to persuade him to stay on.

Byers admitted that he had "made mistakes" but insisted he was not a liar.

He said that Labour and the government were being damaged by him remaining in office.

"People who know me best know I am not a liar," he told journalists.

But he recognised that "there are things I should have done differently."

Byers said he "had become a distraction from what the government is achieving".

"The debate that we need to have about key policy issues is being distorted by my involvement. By remaining in office I damage the government," he said.

"Having worked for the Labour Party all my adult life it is not easy to admit to that reality - but I cannot and will not allow this to continue."

He insisted that quitting was "the right thing to do".

The minister asked his critics to reflect on the part he had played in improving public services.

"I hope people will also remember the part I played in raising school standards when I was at the department of education, the role I played as chief secretary to the Treasury in helping to lay the strong economic foundations we now enjoy," he said.

"I hope they will remember the national minimum wage and the fairness at work legislation that I introduced as trade secretary.

"And I hope they will see that the long term decisions I have taken to restructure the rail industry will stand the test of time."

But he conceded that he had made mistakes.

"Government ministers take many decisions and I know that I have made mistakes; but I have tried to behave honourably and with the interests of the British people at heart," he said

The prime minister defended Byers' policy record but accepted his decision to quit.

Commenting on Byers' policy record, Blair said the minister had "taken the right decisions for the future".

"When he came to me on Monday, he was clear that in the interest of the government he felt he had to leave," said Blair.

"I understand and respect that decision."

But Blair conceded that the troubled minister had "endured a huge amount of criticism".

The prime minister insisted that much of the media's onslaught had been "unfair".

Blair said he was in "no doubt" that Byers would "continue to serve his constituency and the Labour Party".

It emerged on Tuesday that Byers first made his decision to quit last week.

He finally agreed to go during a meeting with Blair last night.

A spokesman for the Conservative party said that Byers had done years' worth of damage to Britain's transport system.

"Stephen Byers has gone but the damage he has done to our roads and railways will last for years. We have been calling for Mr Byers' resignation for months," he said.

"He has been a dreadful minister and has gravely undermined this government's reputation for telling the truth. The prime minister must take the blame for clinging onto this discredited minister for so long. However, Mr Byers' overdue departure will do nothing for our crumbling transport system.

"The real scandal of the Byers era has been the total failure of the government to implement a sensible plan for improving our railways and easing the congestion on our roads.

"The test of his successor will be whether he can get Britain moving again."

The minister's decision to go was welcomed by Lib Dem leader Charles Kennedy.

"Stephen Byers has finally done the honourable thing - and not before time," said Kennedy.

"From any point of view, his position had become untenable.

"Whilst this is a personal tragedy for Stephen Byers, the most important thing is not the fate of an individual minister, but giving the British people a safe and reliable transport system fit for the 21st century."

The prime minister will announce the details of his reshuffle tomorrow.

Downing Street says the reshuffle will be limited - but has not ruled out wider changes to the government frontbench.