The Conservative Party cannot secure a majority unless it wins around new voters by “shouting from the rooftops” about its support for policies such as equal marriage, Minister for Planning Nick Boles has said.

The gay MP for Grantham and Stamford made the remarks in a keynote speech at Westminster for the Conservative think tank, Bright Blue.

Mr Boles said: “While the Liberal Democrats have revealed themselves to be liberal in name only, the frustrations of coalition have at times led the Conservative Party to sound apologetic about its commitment to the expansion of freedom.

“It is a Conservative prime minister who gave gay men and women the freedom to share in one of our greatest and most cherished institutions: marriage.”

Mr Boles said: “The Conservative Party will only win office on its own when it has established beyond doubt its commitment to the advance of freedom and found the courage to stake its claim as the party that all liberals should vote for.”

However, Tory equal marriage critics, such as MP Nadine Dorries, have warned that mentioning the policy in an election campaign would play into the hands of the UK Independence Party.

Mr Boles suggested the Conservatives should revive the National Liberal Party, an organisation that was affiliated to the party from 1947 to 1968.

Sitting MPs would be encouraged to sign up, he said, in an effort to win over voters who might baulk at the idea of calling themselves Conservative in crucial marginal seat fights with the Liberal Democrats.

Downing Street quickly rejected the National Liberal suggestion, but insisted that the prime minister has full confidence in Mr Boles.

Meanwhile, Labour believe Mr Boles’ comments highlight a feeling of frustration that the Tory modernising project remains uncompleted.

Shadow Cabinet Office Minister Jon Ashworth said: “Nick Boles, one of the prime minister’s closest advisers, has let the cat out of the bag. David Cameron’s so-called detoxification of the Tories has been a sham and his modernisation project is dead and buried.”