Germany's centre-left Social Democratic Party (SPD) has overtaken Chancellor Angela Merkel's conservatives after naming former European Parliament President Martin Schulz its leader, according to the latest poll.

The SPD, which has gained around 10 points in opinion polls since the former European Parliament President was appointed in late January, had 32 per cent support, while the Christian Democrats and their Bavarian sister party remained on 31 per cent.

The INSA institute opinion poll published by Bild newspaper found the anti-immigrant Alternative for Germany (AfD) party to be unchanged at 11.5 per cent, while the far-left Linke party was steady at 8.5 per cent and the pro-environment Greens held fast at 6.5 per cent. The pro-business Free Democrats (FDP) lost a half percentage point to fall to 6.5 per cent.

The new SDP leader is already making waves since being unanimously elected by his left-wing party.

The anti-populist has vowed to reject Donald Trump’s “racist and misogynistic” politics and is also known as a staunch critic of Brexit. Last year, he accused David Cameron of “taking a whole continent hostage for a party’s internal struggle”.

He also threatened “the hardest Brexit possible” if MEPs were to be excluded from Brexit talks.

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Social Democrats' general secretary Katarina Barley said the party has seen 13,000 new members join this year.

The SPD’s gains, which are being called the “Schulz effect”, are set to be tested later this month when voters head to the polls in the German state of Saarland.

The vote, the first of three regional elections this year, is seen as a test for the federal election on 24 September in which Ms Merkel is standing for a fourth term.

"This is the first election since the so-called Schulz effect so it will show us whether that really translates into votes for the Social Democrats, and it's an election in which one of Ms Merkel's close confidants is running," said Thomas Jaeger, politics professor at Cologne University.