Labour will not accept a "late, vague plan" for Brexit ahead of voting to trigger Article 50, Keir Starmer has said.
The shadow Brexit secretary warned the Government it must publish its outline for Brexit well ahead of any vote, to allow any amendments to be considered from both sides of the House.
But he was forced to insist Labour did not intend to delay the process beyond the end of March, amid pressure from a number of Tory MPs.
Mr Starmer also faced accusations from Green Party co-leader Caroline Lucas of falling into a Government trap, by supporting its amendment to the Labour motion calling for a Brexit plan to be published.
Mr Starmer told MPs: "The Government must now prepare its plan and publish it.
"And I put the Government on notice, that if it fails to produce a plan by the time we are debating Article 50 legislation – if we are, assuming the Government doesn't win (the Supreme Court appeal) – 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.
"In other words, we're not going to have a situation where the Government seeks a vote in a vacuum, or produces a late, vague plan."
Faced with a revolt by up to 40 Tory MPs, the Prime Minister on Tuesday bowed to pressure and backed a Labour motion which says she should publish a plan before triggering the formal process of leaving.
In return, most of the rebels and Labour will back a compromise Government amendment to support Mrs May's pledge to invoke Article 50 to start Brexit by April.
"The Government has now caved in," said Mr Starmer.
"Last minute amendments tell their own story, and everybody knows it."
Mr Starmer accused the Government of wanting to remove a vote from MPs in its appeal through the courts, and said this motion was important should the Government win, because a plan would be all MPs would have to debate.
He added: "The motion as amended is an indication that the purpose of calling for the plan is not to frustrate the process or delay the Prime Minister's timetable.
"That is what is made clear by the motion and the amendment taken together.
"Labour has repeatedly said that it will not frustrate the process, and I stick by that.
"That's why the Government should prepare its plan and publish it in time for this House to consider it when it debates and votes on the Article 50 legislation."
But his stance was questioned by Brexit Secretary David Davis and other Tory MPs.
Mr Davis said: "Would he please the answer the question as it's put to him.
"Does he think it reasonable, having supported this amendment, that they should frustrate and slow down the process of Article 50?"
Tory Defence committee chairman Julian Lewis added: "If the opposition support or at least do not oppose the Government's amendment today, it would be completely unacceptable and totally inconsistent, having accepted the amendment, then when we get into the new year to do anything to delay the triggering of Article 50 beyond the 31st of March."
Eurosceptic Tory MP Peter Bone (Wellingborough) said there was already legislation in the House to trigger Article 50 before the end of March, with Mr Bone having tabled a private member's bill.
However, Ms Lucas criticised Mr Starmer's stance, saying: "Would he acknowledge that by accepting the Government's amendment to his otherwise very good motion, he's falling into a Tory trap of finding his party in support of invoking Article 50 by March, which is an unrealistic and increasingly arbitrary date."
Mr Starmer said he accepted the Government will not make public all the details and tactics of its negotiating position.
But he said it is essential MPs and the public are given a plan that outlines the starting position.
He said: "Absent of plan, absent knowing the objectives, absent knowing the starting position, the Government would then negotiate for two years without telling us any of that details.
"Is any member of this House content not to know? Hands up who doesn't want to know?"
Tory MP and vocal Brexiteer Jacob Rees-Mogg was the only politician to oblige and stand with his hand up to general laughter.
Mr Starmer added: "It is important there is not a mis-characterisation of this. Asking for a plan setting out the objectives is not to seek to undermine the UK's negotiating plan, nor is it to seek a running commentary.
"But it is, in fact, to have clarity, scrutiny and accountability."
The shadow Brexit secretary said the plan must be detailed enough to "end the circus of uncertainty" that exists on issues such as the single market, the customs union and transitional arrangements.
He added: "That uncertainty is causing more damage to this process than anything else just at the moment."
But Conservative MP and former justice minister Dominic Raab accused Mr Starmer of trying to wreck Brexit.
He said: "I can understand him pressing the Government for its plan, I can understand him setting down his red lines, I can't understand him wanting to enshrine it in legislation.
"The only reason for doing that is so the Labour Party can set the Government up to be sued later. Isn't the truth – will he come clean – it's wrecking tactics by any other name."
Mr Starmer dismissed this assertion, saying: "The answer to the question is no".
He said the plan must also have enough detail to allow the relevant parliamentary bodies to scrutinise it effectively and for the Office for Budget Responsibility to do its job properly.