Support for German Chancellor Angela Merkel's conservative alliance has shrunk to just four percentage points, an opinion poll showed on Sunday - her lowest lead in almost three and a half years.

She maintains a narrow lead over the centre-left Social Democrats (SPD), which a week ago appointed Martin Schulz as leader.

The SDP scored 29 percent in the survey published in newspaper Bild am Sonntag - a six-point jump that Bild said was the biggest pollster Emnid had ever recorded for the party.

The jump took SPD support to its highest in more than four years.

Support for Merkel's CDU and its Bavarian sister party, the CSU, fell four points to 33 percent - cutting the gap between the two blocs to its narrowest in records compiled by poll tracker wahlrecht.de going back to Sept 2013.

Support for German Chancellor Angela Merkel's conservative alliance has shrunk to just 4 percentage points

'Martin Schulz is managing above all to win back former SPD voters and to appeal to them emotionally,' Emnid's Torsten Schneider-Haase told the newspaper, adding: 'Such a strong shift in party preferences within a week is a one-off.'

The SPD appointed Schulz, a former European Parliament president, as leader last Sunday, replacing Sigmar Gabriel who said he was standing aside to boost the party's chances.

The move has re-energised the SPD, junior partner in Merkel's 'grand coalition', ahead of September's federal election.

Schulz has vowed to unseat Merkel with a campaign aimed at overcoming 'deep divisions' that he says have fuelled populism in Germany in recent years.

The SPD appointed Schulz, a former European Parliament president, as leader last Sunday

In a theoretical head-to-head contest for chancellor, the Emnid poll showed Merkel pipping the Social Democrat to victory with 41 percent support to his 38 percent.

The SPD has held exploratory talks with the environmentalist Greens and the far-left Linke party about forming a left-leaning coalition government after the election but they need more support if that is to become a viable option.

The Emnid poll - a survey of 2,233 voters conducted from Jan. 26 to Feb. 2 - showed support for both the Greens and the Linke falling 2 points to 8 percent.

The anti-immigration Alternative for Germany (AfD) was unchanged on 11 percent, with the liberal Free Democrats (FDP) on 6 percent, also unchanged.

Schulz has called for higher wage increases for workers, described U.S. President Donald Trump's policies as 'un-American' and warned against lifting sanctions imposed against Russia over its role in the Ukraine crisis.