Labour has tried to head off a revolt by its own MPs by insisting it is not giving Theresa May a blank cheque to trigger Brexit early next year on her terms.

Keir Starmer, Labour’s Brexit spokesman, denied his party’s backing for a Government amendment to "invoke Article 50 by 31 March 2017” had fallen into a Tory trap.

Instead, Mr Starmer said that backing was conditional on the prime minister producing something better than a "late, vague plan” - or later challenges to the strategy would be made.

The Commons is staging a marathon six-hour debate on a Labour motion to require the prime minister to publish “the Government’s plan for leaving the EU before Article 50 is invoked”.

Ms May was forced to accept the motion, when faced with possible defeat because of a revolt by up to 40 of her own backbenchers.

However, she has split Labour by tagging on an amendment stating that the Commons will also be agreeing to her Article 50 timetable in tonight’s vote.

At least six Labour MPs have said that is a step too far – because No.10 will not say whether its “plan” will consist of any more than broadbrush aims.

Angela Smith, a Yorkshire MP, said she would vote against the motion, because she had “no confidence the Government will not try to wriggle out of the commitment to put a plan before this House”.

But Mr Starmer said, if no detailed plan was produced, “amendments from this side, and possibly from the other side of the House, will be put forward setting out the minimum requirements of a plan”.

He said he “understands concerns” that end of March is too early to trigger Article 50, adding: "Labour does intend to shape the debate and head off a hard Brexit.”

Meanwhile, Brexit Secretary David Davis refused to give MPs a vote on what business leaders see as one of the most damaging potential Brexit decisions – whether to leave the EU’s customs union.

He said the Commons would be informed of the Government’s decision “when we are ready”, with several options still under consideration.

But a Government source said no vote would take place until the end of the two-year Article 50 process – and only if any new customs arrangements required legislation.

Earlier, Labour warned that leaving the union would bring “chaos and gridlock” for companies attempting to obtain supplies from EU countries.

Mr Davis said: “There will be no law changed without the explicit approval of parliament.” However, that commitment only applies to the eventual Brexit deal, The Independent was told.

Pro-Remain Conservative MP Ken Clarke ridiculed the idea that membership of the customs union had been “decided by the referendum” – insisting it was a “mystery to 99 per cent of people" during the campaign

Mr Davis also said it was “inconceivable” that MPs would not be given a vote on the eventual Brexit deal, probably in 2019.