The Lib Dem candidate's tactics have come in for criticism

The campaign pamphlet shows the aftermath of the fatal crash in 2002.

Mr Norris is chairman of Jarvis, which recently accepted joint legal responsibility, but was only a non-executive director at the time.

Mr Hughes has denied using dirty tricks, saying he was just spelling out the choice open to voters.

Mirren endorsement?

Appearing on BBC Radio 4's Today programme, Mr Hughes said: "The question I think London voters must be asked is whether they want a mayor who is in nobody's pocket or whether they want a mayor who has during his candidacy had a direct financial interest in a company that has dealt with the rail service and the tube service and has made a profit from a company doing that."

The Lib Dem MP was also quizzed about an election newspaper published by his campaign claiming he has been endorsed by the actress Dame Helen Mirren.

She has since issued a statement saying she intends to support the current London mayor, Labour's Ken Livingstone.

Mr Hughes said: "That story came from comments she made about her view of moving away from the Labour Party and thinking of supporting the Liberal Democrats."

Campaigns underway

Earlier this week Lib Dem leader, Charles Kennedy, refused to endorse Mr Hughes' pamphlet linking Mr Norris to the Potters Bar rail crash.

The Norris camp meanwhile issued a statement saying: "For the Liberal Democrats to resort to such dishonest and cynical tactics shows how desperate they are.

"Steve will be running a positive campaign on the issues that matter to Londoners, not smears."

The Tory candidate has pledged to give up his work with Jarvis if he is elected mayor.

On Friday Mr Hughes officially launched his campaign for mayor of London later ahead of the 10 June poll.

Among his proposals is a plan to pedestrianise Oxford Street and he wants to give each neighbourhood eight uniformed police officers by 2008.

Hughes' vision

Mr Hughes said: "I want an easier, safer and more united London.

"My vision is nothing less than a Greater London, the most exciting place in Europe and the world, where we are united in tackling crime, terrorism and improving transport."

And in a sideswipe at Mr Livingstone he said: "Blair's mayor has divided communities, divided boroughs and divided London.

"It is time for a new mayor for a Greater London."

Mr Livingstone launched his re-election bid on Thursday with an endorsement from Tony Blair on the same day that Mr Norris officially began his second attempt to win power in London.

The Green Party, whose candidate is Darren Johnson, is publishing its manifesto next Tuesday.