Former Labour MP Denis MacShane, who served as Europe Minister and Foreign Affairs Minister under Tony Blair, said Mr Schulz’s appointment will strengthen Franco-German relations when Theresa May looks to EU counterparts for support.
Martin Schulz, the embattled leader of the Social Democrats (SPD), this week took over as foreign minister, a move likely to infuriate the German voters who opted for more eurosceptic parties in last year’s election.
The former European Union president suffered disastrous results in the German election, but Mrs Merkel’s poor results insured he would be a key part of the coalition, which was finally agreed on Wednesday.
But Mr MacShane warned Prime Minister to be wary.
Writing a piece on LinkedIn, he said: “Brexit for the time being in Britain is now purely an internal Tory party affair with hardly anyone in London bothering about what Europe thinks.
“This political self-regard fill headline in London but leaves the rest of Europe yawning.
“Schulz’s arrival as German foreign minister means that when Britain starts to look across the Channel for support on Brexit it will now face a united Paris-Berlin pro-European axis with the rest of the EU27 falling in behind.”
After the Brexit vote, Mrs Merkel and French President Emmanuel Macron looked to jump start the Franco-German engine once more.
But as an embattled Mrs Merkel struggled to form a government at home following a disastrous election result, relations deteriorated as Mr Macron steamed ahead with plans to further integrate the bloc without the German chancellor.
The appointment of socialist leader, Mr Schulz, therefore means Britain will come up against a strong and united alliance between Germany and France in Brexit negotiations, Mr MacShane warned.
It comes at a time when Mr Schulz peddles the need for a United States of Europe.
Mr MacShane said: “Above all Schulz has aligned himself with Emmanuel Macron, praising the French President’s major EU integrationist speech made at the Sorbonne, Paris in September which has been largely ignored by the British political class.
“Macron phoned Schulz after the unclear election result in Germany and urged him to keep the right-left coalition alive.
“Schulz’s foreign and European policy will now be shaped in coordination with Paris.”
The former Labour politician also took a swipe at the UK’s foreign minister, Boris Johnson, while praising Mr Schulz in comparison, as he added: “Schulz does lead with his mouth at time in the manner of, well, his opposite number in Britain, Boris Johnson, but he has not survived so long in German and European centre-left politics without having an acute sense of what will work and be accepted.”