Martin Schulz, who has emerged as the most serious challenger Angela Merkel has faced in more than a decade in power, is best known in Britain as one of Brexit’s harshest critics.

In his former role as president of the European parliament, Mr Schulz accused David Cameron of “taking a whole continent hostage for a party’s internal struggle” with last year’s referendum, and warned Brexit could result in the “mutual humiliation” of Britain and the EU.

He has threatened “the hardest Brexit possible” if MEPs were excluded from talks.

And in March he was unanimously elected as the leader of the left-leaning Social Democrats, confirming his status as the main opponent to Angela Merkel at the upcoming federal elections.

Now, after a tumultuous week in German politics, he could be leader of Europe’s most powerful country within eight months.

The 61-year-old Mr Schulz is almost certain to lead the Social Democrats (SPD) into September’s elections after Sigmar Gabriel, the current party leader, unexpectedly stepped down.

That decision has turned German politics on its head. One snap opinion poll in the wake of the announcement put Mr Schulz and Mrs Merkel neck and neck, on 41 per cent each.

It is a state of affairs that could see Theresa May pin her hopes on Mrs Merkel clinging on to power, rather than face Brexit’s arch-critic across the negotiating table.