The origin of the £5 billion figure quoted by Stonewall as a possible cost of marriage equality has been revealed.

According to Brian Paddick, the former deputy assistant commissioner of the Met Police and former Lib Dem London mayoral candidate, the figure was calculated for Stonewall by a former civil servant.

The charity is thought to have carried out research into the possible arguments against opening up marriage and civil partnerships to all couples and the figure was cited by chief executive Ben Summerskill at a Liberal Democrat debate on marriage equality this week.

It is understood that the figure relates to a theoretical increase in straight couples taking up the opportunity of civil partnerships, with knock-on implications to their entitlement to pension and tax benefits.

Mr Summerskill was accused by some present at the meeting of arguing against the party’s motion on supporting marriage equality at the Monday meeting. The motion was overwhelmingly passed the next day.

Mr Paddick, who was at the meeting, said the figure was calculated by Stonewall, rather than the government.

He told PinkNews.co.uk: “The Treasury did do an impact assessment for the final reading of the Civil Partnerships Bill.

“And someone who was in the government, actually, who now works for Stonewall, took that assessment and extrapolated it to come up with the £5 billion figure, based on an estimated half a million straight people wanting to enter into civil partnerships if that option was available to them.”

Mr Paddick added that he had been told this by a source within Stonewall.

Mr Summerskill said this evening that the £5 billion figure had been extrapolated from the final regulatory assessment of the Civil Partnership Bill and had been calculated by a former civil servant for Stonewall, as the original assessment made no mention of civil partnerships for heterosexuals.

He added that it was “ludicrous” to ask at 7pm on a Friday evening whether that figure was based on an estimate of half a million straight people entering into civil partnerships if they were allowed to.

“Our clear argument is that if people are told that the government will introduce [an equality measure] that may cost £5 billion over ten years, they may be skeptical,” he said.

“Supporting straight people is not one of our objectives. They can look after themselves. It is an issue of strategy and tactics.”

This evening, Stonewall announced that it was “consulting widely” on the future of civil partnerships and said it hoped its final position had the support of the wider gay, lesbian and bisexual public.

The charity, which used PinkPaper.com to publish its statement, said: “We’re consulting as widely as possible on the future of civil partnership and gay marriage.

“There are a number of different options which all have vocal supporters and detractors. That’s why we’re determined to try and build a broad consensus in the LGB community so we can be sure that our final position has the backing of Stonewall supporters and the wider lesbian, gay and bisexual public.

“We’re also determined that this issue shouldn’t be used as a party political football.”