David Mundell

The secretary of state for Scotland, who is the first openly gay Tory cabinet minister, described his announcement as "one of the most important decisions of my life".

"How can it be both so easy and so hard to say a few short words?" he wrote in a message on his website. "In the end, it took just a couple of taps on a keyboard, yet at some points, in my mind, it was going to be harder than standing for election, speaking in the House of Commons or being cross-examined on television.

"I still cannot fully rationalise such feelings, but I know they are not uncommon, particularly in men of my age."

Scotland's only Conservative MP has a strong record of voting for the extension of LGBT rights, including same-sex marriage. Originally a member of the Scottish parliament, the 53-year-old has represented the Westminster constituency of Dumfriesshire, Clydesdale, and Tweeddale since 2005 and has three adult children.

He has served in David Cameron's cabinet as secretary of state for Scotland since the 2015 general election.

"Gender and sexuality should make no difference whether you are a cabinet minister or in any other walk of life and I hope that I can, in my own way, reinforce that message," he concluded.

A Downing Street spokesperson said David Cameron had been told of Mundell's decision "in recent days".