Anna Soubry said the Brexit delay revelation in court documents shows the untrustworthiness of Boris Johnson.
The Independent Group for Change leader said: "I just think this is further evidence that you can't trust a single word that this Prime Minister says.
"It's really serious because he obviously said he would rather die in a ditch or some silliness than hand over this letter, whereas these papers prove he is absolutely going to deliver that letter.
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"And he also anticipates, in my view, that we won't have a deal on 19 October and now this is really serious stuff.
"You have to say to yourself 'what can you believe from this man?' and I think it casts real doubt on whether he has any intention whatsoever to get a deal with European Union by the 19th."
Ms Soubry said she does not believe there is a chance of no-deal on 31 October because the Benn Act is "watertight".
In documents submitted to the Court of Session, lawyers representing Mr Johnson said the Prime Minister would respect the terms of the Benn Act, which calls for Brexit to be delayed if a formal agreement has been reached.
He had previously said he would rather "die in a ditch" than request a second extension to the Brexit deadline.
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Andrew Webster QC, representing the UK Government, said the documents it had submitted to the court were a "clear statement" as to what the Prime Minister would do.
He argued there was no need for an order to be made forcing a letter requesting an Article 50 extension to be sent under the terms of the Benn Act, because the court has it on record it will be sent.
He said: "What we have is a clear statement on behalf of the Prime Minister and Government as to what it will do in respect to the requirements of the 2019 Act.
"They have been put on record so there can be no doubt."