Enshrining the date of Brexit into law could cause "significant difficulties" in talks with Brussels, a cross-party committee of MPs has warned Theresa May as she continues to face pressure to abandon the plan.
The Government wants to amend the EU (Withdrawal) Bill to include the departure date of 11pm on March 29, 2019 and the prime minister has said she will not back down over the demand.
But the Commons Exiting the EU Committee said this would take away ministers' "flexibility" if the negotiations went down to the wire.
The committee also warned that the provisions in the Bill to convert EU law into British law raised "significant legal and constitutional questions".
However, the report split the committee, with four of the eight Conservative MPs present and the single Democratic Unionist voting to reject it, something which could dilute its impact.
It follows a report in the Telegraph newspaper that the Government is considering removing the amendment. The proposal has proved controversial, with more than a dozen Tory MPs threatening to vote against it.
The Telegraph reported that as many as 27 Conservative MPs are prepared to oppose Mrs May over the amendment, which could see the Government lose any vote in the Commons.
In its report, the Commons Exiting the EU Committee said that, as it was originally drafted, the Bill gave ministers the flexibility to set multiple exit dates allowing for the setting of "different commencement dates for different provisions", as well as providing for possible transitional arrangements.
"The Government's latest amendments will, however, if agreed by the House, remove this flexibility by setting the exit day in the Bill as March 29 2019 at 11pm," the report said.
"This would create significant difficulties if, as the Secretary of State (David Davis) suggested to us in evidence, the negotiations went down to the 59th minute of the 11th hour."
The finding was opposed by Conservative MPs Jacob Rees-Mogg, Peter Bone, Christopher Chope and Craig Mackinlay as well as the DUP's Sammy Wilson.
Brexit Secretary David Davis has indicated that a compromise might be in the offing over the plan, but the prime minister's spokesman said it was important to provide certainty about the date of the UK's departure from the bloc.
"As the PM has said, we want to listen to Parliament," her spokesman said.
"We are clear, though, on our determination to get the best possible outcome for the UK. Key to doing that is certainty.
"What the amendment does is provide certainty over our position that we are leaving the EU on March 29 2019. We would encourage all MPs to support it."