The Prime Minister sacked Gavin Williamson earlier this month (Picture: PA).
Former defence secretary Gavin Williamson has described Theresa May's cross-party Brexit talks with Labour as "a grave mistake" and a "betrayal of democracy".
Mr Williamson, who was sacked over the Huawei leak, said pressing ahead with the talks will have "fatal" consequences.
Writing in the Mail on Sunday, he labelled the Prime Minister as "politically naive" for going into fruitless negotiations which he claimed were bound to fail.
"Jeremy Corbyn will do all he can to divide, disrupt and frustrate the Conservatives in the hope of bringing down the Government," he wrote.
Mr Williamson said Theresa May seemed oblivious to the fact that many Tories believe she is "negotiating with the enemy".
The former Cabinet minister said there was a simple calculation that a deal could pass with the combined votes of Labour and Conservative MPs, but "tough realities" must be faced if the deal was "far removed" from expectations.
Warning Mrs May she was turning her own supporters against her, Mr Williamson said that scenario "should be avoided at all costs":
"It is a grave mistake for any Prime Minister to fail to recognise when a plan will not work and it is fatal to press on regardless."
Mr Williamson's comments come as two new opinion polls reveal a big lead for Nigel Farage's Brexit Party ahead of the European Elections.
Labour is second in the polls and the Conservatives fourth.