BERLIN (Reuters) - German Chancellor Angela Merkel’s conservatives and the center-left Social Democrats (SPD) have agreed to impose a news blackout during exploratory talks next week on forming another ‘grand coalition’, Der Spiegel magazine said on Thursday.

A tie-up between Merkel’s bloc, comprising her Christian Democrats (CDU) and their Bavarian CSU sister party, and the SPD is her best chance of securing a fourth term after efforts to form a coalition with two smaller parties failed last year.

The breakdown of those talks in November was widely blamed on politicians leaking information from meetings, eroding trust between prospective partners.

Merkel, CSU leader Horst Seehofer and SPD leader Martin Schulz have agreed that negotiators should not be allowed to give interviews or appear in television talk shows during the exploratory talks, Der Spiegel reported.

Both sides agreed that no advance word of a possible outcome should be leaked during the talks to shore up trust among the negotiators, the magazine added.

Germany’s largest parties on Wednesday expressed optimism about the chances of forming a government after several hours of talks.

The parties talked of growing confidence - good news for Merkel, whose reputation as Europe’s consummate consensus-builder is on the line in this second bid to form a coalition government after weeks of sniping between its would-be partners.

The exploratory talks are due to start on Sunday and could lead to official coalition negotiations in a few weeks.

To keep its rank and file on board, the SPD leadership has said it will let its members vote on Jan. 21 on going ahead with detailed coalition talks after the first phase has been completed.

Merkel’s arch-conservative CSU allies meanwhile warned the SPD not to make too many demands in coalition talks but were keen to show their core voters that there would be no compromise on sensitive issues like immigration.

“We want this coalition,” CSU leader Seehofer told a news conference ahead of a parliamentary caucus meeting. “The project can succeed if the potential partner doesn’t overbid.”

Seehofer added the CSU’s proposals to tighten immigration and asylum laws were not meant to upset the SPD. “It’s not aimed against anybody. It’s rather about stating our positions clearly,” Seehofer said.

“This project (of another grand coalition) can succeed if our potential coalition partner doesn’t take it too far,” he said.

Merkel is under increasing pressure to reach a deal with the SPD, which also lost ground in the September election, but the two sides have bickered over a range of issues, notably immigration and taxation.