Tony Blair has been accused of “unacceptable” behaviour after it emerged he has been briefing Emmanuel Macron on how to force Britain to stay in the EU.

The former Labour prime minister believes that if the EU stands its ground over the Brexit deal, Parliament will cave in and accept a customs union - which would keep Britain yoked to Brussels - or a second referendum that could cancel Brexit altogether.

Sources in Paris confirmed to The Telegraph that Mr Blair had been speaking to the French President about Brexit.

He is reported to have told Mr Macron to “hold firm” and wait for events to play out in London that end in Britain staying in the EU.

Peter Bone, a Eurosceptic Conservative MP, said: “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. I doubt he would have liked it if his predecessors had done this to him.

“Thankfully I don’t think it will have any effect other than to damage his legacy.”