Luxembourg's foreign minister says there is "no better" Brexit deal for Britain than the one that was finalized last week.
She suggested that agreeing on conditions favorable to Britain could satisfy concerns of her critics.
The future of Brexit seems uncertain now, but as a demonstration effect, the problems Britain is facing to leave the European Union might have prompted the other European Union members thinking of following Britain out of the bloc to give it a second thought.
Raab said that while the agreement was "fatally flawed", it's not too late to change that.
He added: "I find it impossible to believe that the Secretaries of State of exiting European Union, whether it's David Davis or Dominic Raab, were not aware of the sort of direction in which this negotiation was going".
May also insisted that MPs bidding to topple her did not have the 48 names needed to trigger a confidence vote in her leadership.
Following strong criticism of her exit deal, May used an interview on Sunday to emphasise the outline agreement on Britain's future relationship with the bloc was still being negotiated and would deliver on the 2016 Brexit vote.
How close is May to a no confidence vote?
Asked if the 48 figure has been reached, May said: "As far as I know, no it has not".
"It's critical that people trust my integrity in this", he said.
Theresa May is also the threat of a vote of no confidence by the members of his party.
At such a critical moment in politics - when the draft Brexit agreement hangs in the balance and the PM's future is being openly questioned - you feel an even greater responsibility than usual to get the questions right. The task ahead was hard.
The turnaround was remarkably quick in 2003, with reports emerging that the 25 letters had been submitted on 28 October and Duncan Smith voted out the following evening. Several ministers, including her Brexit minister, have resigned and some of her own members of parliament are seeking to oust her.
Mrs May said negotiations were continuing and she meant to go to Brussels and meet European Commission President Jean-Claude Juncker. Some Brexit-supporting ministers are reported to want to rewrite parts of it, though Germany has ruled this out.
Mrs May replaced McVey with ex-home secretary Amber Rudd, while the little-known Tory MP Stephen Barclay has replaced Raab.
Mr Raab, who negotiated for the United Kingdom with the EU's Michel Barnier, has been explaining his decision to quit on the BBC's Andrew Marr show. He said May must change course. "It would not. That issue is still going to be there", she told the Daily Mail newspaper.
Even if May sees off such a challenge, she still has to get the deal approved by Parliament.
"Instead of a system based on where a person is from, we will have one that is built around the talents and skills a person has to offer", she said, adding that the core of the post-Brexit immigration system will be "skills based" rather than "quota based".
'I do think there is a point at which, we probably should have done it before, where we just say "I'm sorry, this is the United Kingdom of Great Britain and Northern Ireland, we can not accept those dictated terms"'.