In a speech to MEPs he said unless the Irish border row is resolved the deal is "zero per cent" finished as he mocked May's estimation.
She told parliament that 95% of the terms of exit were agreed but the Irish border was still a "considerable sticking point".
"Taking all of this together, 95% of the withdrawal agreement and its protocols are now settled".
As May said in her speech, she rejects the EU proposal for Northern Ireland to remain in a customs union with the bloc, as that could make trade with the rest of Britain more hard, something that is opposed by the Pro-Brexit Northern Irish party DUP that has propped up her minority Conservative government.
Speaking at a House of Commons EU Scrutiny Committee meeting, the minister said there is no "cast-iron timetable" for when an adequacy agreement will be in place.
At the summit in Brussels, May offered further compromises, pledging to consider extending the transition period and to drop her demand for a strict end-date to the so-called backstop arrangement for the Irish border.
But the deal - the terms of Britain's divorce - can not be signed off until the two sides settle on future management of the border between Northern Ireland and European Union member state Ireland.
Her statement came just hours after arch-Brexiteer Steve Baker scrapped an attempt make a Brussels plan to keep Northern Ireland in the EU's customs union after Brexit illegal under United Kingdom law.
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That would mean it did not automatically have the right to reclaim its current terms of membership, such as the generous budget rebate negotiated by ex-UK prime minister Margaret Thatcher in 1984, should it want to rejoin.
Philippe Lamberts, a Belgian Green MEP, told Business Insider that Westminster MPs who want to stop Brexit should support any deal the prime minister brings back from Brussels, in order to avoid the chaos and disruption of a no deal scenario and create a longer window in which they could push for another referendum.
Prominent Brexiteer Steve Baker, a former junior Brexit minister, will try to block the EU's backstop plan on Wednesday by attaching amendments to legislation passing through parliament.
Any such policy must meet the test of delivering our safe exit from the European Union, thereby honouring the referendum result, and do so in a way that does not harm British industry.
British Prime Minister Theresa May on Monday used a speech to Parliament to try and reassure lawmakers that an agreement on Britain's withdrawal from the European Union was nearly complete, amid growing rebellion against her plans for Britain's future relationship with the bloc.
Several Sunday newspapers said rebellious MPs were preparing a fresh bid to topple her leadership this week, many carrying colourful off-record quotes from the plotters. But at no stage has the Prime Minister ever reached out to Parliament or sought to build a national consensus on our future relationship with the EU.
It is not clear if Mr Raab's comments signal a clear split between Mrs May and her Brexit secretary on the issue.
Ms Flint told The Yorkshire Post: "There are a number of Labour MPs who feel if there's a reasonable deal on the table, if you're going to say no you need some bloody good reasons why if we're going to end up with no deal".