The government is under pressure from rebel Tory MPs and Labour to guarantee a Commons vote on Brexit before the UK leaves the EU in March 2019.
David Davis denied “fudging” the commitment by suggesting on Wednesday such a vote could happen after that date if talks go to the wire.
He said he “fully expects” MPs to get a “meaningful” vote in time.
But ex-minister Nicky Morgan told him Tory rebels were “deadly serious” about getting the promise into law.
It comes as the government announced that MPs would begin detailed scrutiny of the EU Withdrawal Bill on 14 and 15 November.
Former education secretary Ms Morgan said the government could put the issue of a vote “beyond doubt” by backing an amendment to the EU Withdrawal Bill, which was tabled by former attorney general Dominic Grieve.
Citing reports that Mr Davis did not think rebel Tories were serious about the amendment, she said: “Can I tell him we are deadly serious, and it would be better for the government to adopt a concession strategy on having a withdrawal agreement secured by statute sooner rather than later for all concerned.”
In reply, the Brexit secretary said: “I won’t pre-empt the discussions in the bill, but those reports are not true.”
For Labour, Keir Starmer said: “More than three months after the repeal bill was first published, the government has finally found the courage to bring it back to Parliament for line-by-line scrutiny and debate. Whether it will survive scrutiny remains to be seen.”
He added: “Ministers must listen to the sensible demands made by Labour and some of their own MPs about what changes need to be made to the bill.”
David Davis was summoned to the Commons on Thursday morning to explain comments to the Brexit committee on Wednesday, which suggested the vote could take place after March 2019.
At PMQs later on the same day Theresa May had to reassure MPs there would be a vote.
Sir Keir Starmer accused the government of saying “one thing one day, another thing the next”, which he said was “not good enough”. He called for a “cast-iron guarantee” that MPs would get a vote.
Mr Davis denied any government commitments had been undermined by his comments, which he said had been misinterpreted.
He said he expected a “proper and meaningful” vote on any Brexit deal “at the right time”.
But he added: “Of course this vote cannot happen until there is a deal to vote upon, but we are working to reach an agreement on a final deal in good time before we leave the European Union in March 2019.
“Clearly we cannot say for certain at this stage when this will be agreed, but as (EU chief negotiator) Michel Barnier said, he hopes to get a draft deal agreed by October 2018 – and that’s our aim as well.
“So we fully expect there will be a vote in the UK Parliament on this before the vote in the European Parliament, and before we leave the European Union.”
He also insisted the choice for MPs would be “meaningful – whether to accept that deal, or to move ahead without a deal”.
Referring to his appearance before the Commons Brexit Committee, Mr Davis said the government recognised the ratification of the Brexit agreement “will take time and could run into the implementation period”.
He denied claims he had “fudged” the government’s commitment to a vote before Brexit but added that he was not in control of the timetable of Britain’s exit from the EU because “it’s a negotiation”.