Conservative MP Stephen O’Brien, who is also the Prime Minister’s Envoy & UK Special Representative to the Sahel region in Africa, has confirmed he will vote against equal marriage because of a lack of “detail” in the government’s plans.

In a letter to a member of the public, the MP for Eddisbury in Cheshire, said:

I am personally troubled that if it is being proposed on equality grounds, it would in my view need to be accompanied by the right of heterosexual couples to be able to enter into a civil partnership should they wish.

If it is for ‘commitment’ reasons, I remain very concerned at the inconsistency with, say, two siblings deciding to live together under the same roof over a long period of time.

If it is to be consistent with ‘marriage,’ in my view there will need to be unjusticiable clarity about the concept of annulment/consummation, adultery and grounds for divorce just as there are currently for heterosexual married couples.

Lawyers have warned the UK’s laws on adultery and divorce may have to be amended to include gay couples, as part of the government’s equal marriage reforms.

On the question of heterosexual civil partnerships – which Mr O’Brien mentioned in his letter – human rights campaigner Peter Tatchell recently said the government’s decision not to include the provision in its reforms meant “gay couples will have a legal advantage, if straight couples are not offered the right to civil partnerships.”

Mr O’Brien served as Parliamentary Under-Secretary of State for International Development until September’s reshuffle – when he was replaced by Lynne Featherstone, the Lib Dem former equalities minister.