"Dreaming of a world that had turned out differently is not enough", former Conservative leader William Hague, an influential voice in the party, wrote in The Daily Telegraph. It is an indulgence not a policy'.
He has also criticised Downing Street for briefing Labour MPs about its Brexit plans, describing such a development as "a matter of grave concern".
What has been outlined so far has sparked a backlash from Brexiteers and prompted the resignations of David Davis and Boris Johnson.
Unnamed Brexiteers were quoted in the press as saying the PM should brace herself for "guerrilla war" to kill off her deal, with Eurosceptic Tories withholding their support in the Commons.
Bernard Jenkin said the European Research Group - led by influential Eurosceptic Jacob Rees-Mogg - did not expect more resignations and backed Mrs May's leadership.
On Thursday, the government proposal or White Paper was due to be published, setting out the terms for Britain's exit from the European Union and its future relations with the bloc after March 2019.
All eyes are now on the next move by Brexit hardliners in her centre-right Conservative Party.
A year later, commentators said recent events show she has firmly chosen the moderate side with a plan for Brexit, to be fully detailed today, that envisages close regulatory alignment with the European Union to allow unhindered trade in goods.
They both accused the prime minister bluntly of pushing such a bad version of Brexit that it would help get Jeremy Corbyn into Downing Street.
In the foreword to the 120-page document, new Brexit Secretary Dominic Raab will say it is "a vision that respects the result of the referendum and delivers a principled and practical Brexit".
The strategy announced after Friday's meeting at the PM's Chequers country retreat came after months of cabinet divisions.
Hard Brexiters have pledged to mount a campaign of guerrilla warfare against May's government in an attempt to disrupt the soft Brexit proposal unveiled at Chequers, in which she proposed that the United Kingdom would share a "common rule book" of standards on food and goods after Brexit.
"We need to see whether the taskforce can accommodate some of the British thinking", he said.
Brexit betrayed or the end of May: What does David Davis's resignation mean?
"There is a whole range of scenarios in which they get no Brexit, or an indefinitely delayed Brexit, or a change of government or a second referendum".
"It would be hard to say at this moment what the consequences would be, but the consequences would be very serious for delivering Brexit", he said.
"It's a very confident country, it's a very capable country. It is an indulgence not a policy".
'The Chequers deal is a plan that has been put together, it's been agreed by government, ' she said as she arrived in Brussels for the North Atlantic Treaty Organisation summit.
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