BERLIN, Nov 27 (Reuters) - German Chancellor Angela Merkel's conservatives and the centre-left Social Democrats (SPD) made a breakthrough early on Wednesday in talks about forming a "grand coalition" government, two top conservative politicians said on their Twitter accounts.

Germany's two biggest political forces negotiated through the night and reached a breakthrough at about 5 a.m. (0400 GMT), said senior Merkel lawmaker Michael Grosse-Broemer and Annegret Kramp-Karrenbauer, the conservative premier of Saarland state.

Two months after Merkel's election victory and a month after coalition talks began, the agreement, if confirmed, would enable her to firm a government by Christmas - if the SPD gets approval in a vote among more than 474,000 party members.

The party leaders are expected to present details of a deal at a news conference on Wednesday, the deadline set by Merkel. But they may wait two more weeks to announce the allocation of cabinet posts.