Martin Schulz, the controversial former president of the European parliament, became the favourite to take on Angela Merkel in German elections after his party leader pulled out in a shock decision on Tuesday.

The decision means that one of the EU's most outspoken critics of Britain over Brexit could become German chancellor at a crucial point in negotiations this September.

Sigmar Gabriel, the current vice-chancellor, told a meeting of the centre-left Social Democratic party (SPD) on Tuesday afternoon that he would not seek its nomination as a candidate for the chancellorship.

Instead he said he would resign as party leader and recommend Mr Schulz to succeed him.

“There are two basic requirements to lead a successful election campaign: the party must believe in the candidate and gather behind him, and the candidate himself must want it with every fibre of his heart,” Mr Gabriel told Germany’s Stern magazine in an exclusibe interview

“Neither of these apply to me in sufficient measure.”