The Forsa poll for the weekly magazine Stern and broadcaster RTL put Merkel's Christian Democrats (CDU) and their Christian Social Union (CSU) Bavarian sister party on 38 per cent, up from 36 per cent a week ago. The SPD fell three points to 26 per cent.
Forsa carried out the poll before an important state election on Sunday won by the CDU. Other polls have put the conservatives about 9-10 percentage points ahead of the SPD nationally.
The latest Forsa poll adds to expectations that the conservatives will win the September 24 federal vote. Merkel, first elected in 2005, will be seeking a fourth term.
The SPD, environmentalist Greens (7 per cent) and the hard-left Die Linke (8 per cent) together have 41 per cent.
The nationalist Alternative for Germany (AfD) party, which has no political allies, is on 7 per cent.
The poll was conducted May 8-12, included 2,506 respondents and had a margin of error of +/- 2.5 per cent.
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)
Get the latest election news, live updates and election schedule for Lok Sabha Elections 2019 on ndtv.com/elections. Like us on Facebook or follow us on Twitter and Instagram for updates from each of the 543 parliamentary seats for the 2019 Indian general elections.