Tony Blair has been advising Emmanuel Macron about how to keep Britain in the European Union, it has been reported.
The newspaper said sources in Paris had confirmed that Mr Blair has been talking to Mr Macron about Brexit.
Mr Blair is reported to have told Mr Macron to “hold firm” while events play out in the UK.
Parliament is set to vote on prime minister Theresa May’s Withdrawal Agreement on Tuesday.
Mr Blair is reported to have told Mr Macron that Parliament may eventually accept a customs union or grant the British public a second referendum on Brexit.
Eurosceptic Tory MP Peter Bone told The Telegraph: “It is totally unacceptable for a former prime minister to go around the heads of European countries and undermine the government’s position.
“I’m not sure we could find another time in modern history when this has happened.
“Tony Blair has to realise he has had his time as prime minister and I’m surprised and disappointed he has done this.
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“I doubt he would have liked it if his predecessors had done this to him.”
Mrs May was working down to the wire to find a Brexit breakthrough before Tuesday’s crucial vote by MPs on her Withdrawal Agreement, with speculation mounting of last-minute talks with Jean-Claude Juncker.
Ireland’s foreign minister Simon Coveney said he understood that the Prime Minister will travel to Strasbourg on Monday evening in a final bid to secure changes that will allow MPs to back her Withdrawal Agreement the following day.
But sources in London said the Tanaiste was “getting ahead of himself” and there were no confirmed plans for Mrs May to fly to the continent.
Neither Downing Street nor the European Commission would confirm plans for a face-to-face meeting between Mrs May and Mr Juncker, who spoke by telephone on Sunday evening and again on Monday.
Time is running out for any new assurances or clarifications to the deal which was resoundingly rejected by a 230-vote majority by MPs in January.
The Government must table its motion for Tuesday’s debate by the end of the day, alongside the publication of any relevant documents – including Attorney General Geoffrey Cox’s legal advice on the deal.
Downing Street insisted following Monday’s phone call that “talks continue”, despite admitting the process was “deadlocked” after negotiations over the weekend failed to produce agreement.
European Union chief negotiator Michel Barnier said the talks happening now were “between the Government in London and the Parliament in London”, rather than the UK and Brussels.
At Westminster, speculation has centred on the prospect of the Prime Minister watering down her commitment to hold a vote on her Brexit deal following the failure of talks to provide suitable concessions over the Northern Ireland backstop.
But Mrs May’s official spokesman confirmed it remains the plan to stage the second “meaningful vote” on the Brexit deal on Tuesday.