The Deputy Prime Minister Nick Clegg has previously ridiculed the idea of a tax break for married couples as "patronising drivel that belong in the Edwardian age".
The coalition agreement spells out that Liberal Democrat MPs can abstain on a plan to recognise marriage in the tax system.
More top news
The High Court will hear a legal challenge to Labour's decision to guarantee Jeremy Corbyn a place on the party's leadership ballot.
Russian Foreign Minister Sergei Lavrov brushed aside accusations that Moscow was behind the hacking of internal Democratic Party memos.
The First Lady gave an enthusiastic backing for the Democratic presidential frontrunner in a speech at the Democratic National Convention.