Former Brexit Secretary Dominic Raab has criticised the government's "lack of political will and resolve" in dealing with the European Union.
Mr Raab, who quit on Thursday over the Brexit deal, told the Sunday Times the UK should not allow itself to be "bullied", and must be prepared to walk away from negotiations if necessary.
There has been widespread criticism of the PM's draft withdrawal agreement.
However, Theresa May dismissed suggestions the deal could be amended.
Commons leader Andrea Leadsom, believed to be one of five cabinet ministers hoping to persuade the prime minister to rethink her plan, has said there is "still the potential to improve" the agreement.
But Mrs May, writing in the Sun on Sunday, insisted again it was the only deal that would work.
"There is no alternative plan on the table. There is no different approach that we could agree with the EU," she said.
The publication of the draft withdrawal agreement on Wednesday prompted the start of a tumultuous few days for Mrs May, with two senior ministers and several other junior ministers and aides resigning.
Some Conservative Brexiteers who are unhappy with it have also been submitting letters of no-confidence in Mrs May.
'Afraid of our shadow'
Mr Raab said if a deal could not be closed "on reasonable terms we need to be very honest with the country that we will not be bribed and blackmailed or bullied, and we will walk away".
He also warned against looking "like we're afraid of our own shadow".
"I think there is one thing that is missing and that is political will and resolve," he said. "I am not sure that message has ever landed."
BBC political correspondent Iain Watson said Mr Raab was careful not to attack the PM directly in his newspaper interview.
The 585-page draft document sets out the terms of the UK's departure, including details such as how much money will be paid to the EU, details of the transition period and citizens' rights.
Both the UK and the EU want to avoid a hard Northern Ireland border so they agreed to include in the deal a "backstop" - or back-up plan - in case they cannot reach a long-term trade agreement which does this.
This would mean Northern Ireland would stay more closely aligned to some EU rules, which critics say is unacceptable.
And the UK would not be able to leave the backstop without the EU's consent.
The former Brexit secretary is not one of the Conservative Brexiteers to have submitted a letter of no-confidence in Mrs May.
If 48 letters are sent, then a vote will be triggered and she could face a challenge to her leadership.
Tory MP Zac Goldsmith, the party's London mayoral candidate in 2016, has revealed he has sent a letter.
Writing in the Sunday Telegraph, the Brexiteer said that under the PM's plan "in effect, Britain would remain in the EU, but without having any say".
He added: "Had that been the choice, I personally would have voted to remain."
MPs will get the chance to vote on the deal in Parliament, after it is approved by the EU next Sunday, 25 November.