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
No EU national currently living lawfully in the UK will be made to leave on the day of Brexit under proposals outlined by Theresa May.
A revolutionary cancer-fighting machine will deliver proton beam therapy for the first time in the UK.
Low pressure brings cooler, more seasonal weather over the next few days.