It was the big number in Monday's front-page stories: Downing Street's warning that the bill to legalise gay marriage could cost an extra £4bn.
This, it was claimed, is how much a Tory backbench amendment would cost which would allow straight couples to enter into civil partnerships, thereby equalising treatment between the two groups. Labour and others scoffed at the total.
So is the £4bn figure accurate?
It is possible that allowing straight couples to enter civil partnerships would cost a significant amount. One of the reasons why gay rights campaigners want access to marriage is because it offers more generous pension entitlements.
The £4bn figure was first touted last week by the DWP pensions minister Steve Webb, who believed the liability would follow as the government would be forced to change the pensions rules. These currently discriminate against those in civil partnerships. If the positions were equalised, couples currently in civil partnerships would benefit from uprated pensions should their partner die. In effect it would be untenable under pensions rules for some straight couples in civil partnerships to get different treatment from straight couples in marriages.
If this change occurred, all those currently in civil partnerships – who the government is assuming would not otherwise bother to convert their legal status to marriage – would also end up getting better pension settlements when their partners die.
But is the £4bn figure right? No: on Monday afternoon the Treasury admitted the figure had nothing to do with any actual estimation of costs that the amendment would create. Although the understanding of how the liabilities could arise is right, a Treasury spokesperson said the figure was meant to be "illustrative" only.
The costing was taken from government evidence in the 2011 legal battle Cockburn v the secretary of state for health, which, although it was about pensions, surviving spouses and equal treatment, had nothing to do with civil partnerships.
"[It] is a figurative example that the minister uses to exemplify what happens when you start to run into equalising rights for people. There are costs to [doing] that," a Treasury spokesperson said.
