Cabinet Office minister Francis Maude is expected to warn today that the Tories must modernise and approve measures including equal marriage or risk being seen as ‘unacceptable and unelectable’.

In a speech due to be given to the Policy Exchange think tank tonight, he will say: “We are the party that sent the first female MP to Westminster; and we are of course the party of the first female Prime Minister. We legislated to introduce women peers into the Upper Chamber and in government now we will legislate to introduce same-sex marriage.

“But we have also been on the wrong side of the debate – and when we were and when we are again we must have the courage to say so, to accept that we were wrong and to change.

“The Conservative Party will always suffer if it is seen as if it is trying to turn the clock back to an imagined golden era. You can’t drive policy looking through a rose-tinted rear-view mirror. If we are seen as being defined by backward-looking social attitudes we will be seen as unacceptable and unelectable.”

He will add: “We shouldn’t arrogantly assume we always know best and that society should conform to our expectations rather than us adapting to evolving social norms.”

Mr Maude is also expected to talk about the death of his gay brother from AIDS in the 1990s.

In 2006, he told PinkNews.co.uk the anti-gay policies the Conservatives presented to the country during the 1980s and the 1990s, were “wrong”.

He told PinkNews.co.uk that times had “moved on and the Conservatives should have moved on with it much, much earlier and we didn’t. A big part of our problem more generally was that we have failed to keep pace with change in society.”

On gay partnerships, he added: “For me its all to do with family values it is better for society that we should recognise those couples who wish to make a long term commitment to share responsibilities. It’s about strengthening our society.

“This is all informed by my family, my wonderful, intelligent, beloved brother. The gay scene in London in the 1980s was quite aggressively promiscuous and I think if society generally and the government I served in had been more willing to recognise gay people then there would have been less of that problem.

“A lot of people like my brother would not have succumbed to HIV and lost their lives.”

Mr Maude voted in favour of allowing gay couples to adopt children in 2002, defying a three-line whip imposed by the then leader Iain Duncan Smith.

He is expected to add in his speech today that the Tory party is a “phoenix not a dodo”, saying: “To survive and succeed over the centuries it has had continually to modernise and evolve. When we have failed to do so – when we failed to understand and influence the spirit of the age – the electorate rightly punished us.”