The deal to end the Tube dispute brokered by London Mayor Ken Livingstone could be challenged by legal action.

Unions called off further strikes after Mr Livingstone offered to take the dispute to independent arbitration when he takes responsibility for London Underground early next year.

But Tim Collins, shadow transport secretary, said he was consulting lawyers about action against the mayor.

He said he believes Mr Livingstone's promise to back-date any pay award was "potentially illegal".

We will be consulting to get advice on whether Ken Livingstone has acted outside his powers

Tim Collins MP, shadow transport secretary

Leaders of the Rail Maritime and Transport (RMT) union and Aslef say they expect large pay rises for Tube workers, which the mayor said could be back-dated to April this year.

But Mr Collins said back-dating a deal to a period when Mr Livingstone was not in charge was "unethical and potentially illegal".

He said at the Conservative Party conference in Bournemouth that members of the Society of Conservative Lawyers were being consulted to see if the offer could be blocked.

"We will be consulting to get advice on whether Ken Livingstone has acted outside his powers," he said.

The unions staged two 24-hour walkouts in September, causing massive travel disruption.

Control changes

It is thought the stoppages cost the economy �125m.

Transport for London (TfL), the mayor's transport body, is expected to take control of London Underground early in 2003 as the public-private partnership deal is completed.

Mr Livingstone said on Wednesday night he wanted transport commissioner, Bob Kiley, to negotiate a four-year deal "to bring industrial peace to the Underground".

'Way forward'

"This strike has been very much about management opposing a settlement and ignoring the agreed procedure.

"It will be ridiculous to allow strikes to continue week in and out for months to come when there is a way forward."

London Underground said it planned to go ahead with talks with the unions later this week.