Conservative Party leader David Cameron has been urged to sack a shadow minister who disagreed with the age of consent for gay men because it poses an “HIV risk”.

Dr Julian Lewis, the incumbent MP for New Forest East and the shadow defence minister, wrote a letter to a constituent in which he argued that the age of consent for gays should not have been equalised in 2000.

In the letter, first seen by PinkNews.co.uk, Dr Lewis, who is standing for re-election, wrote that there was a “seriously increased risk of HIV infection arising from male homosexual activity”.

He added: “When it comes to legalising practices that involve serious risk, I believe the higher limit should apply. This is the reason we no longer allow 16 and 17-year-old into front-line situations in the Armed Forces, for example.”

Last night, home secretary Alan Johnson called on Mr Cameron to sack Dr Lewis.

Mr Johnson wrote: “You have been actively seeking the votes of gay people throughout Britain, but your frontbench team includes people who are evidently against any notion of homosexual equality.

“You need to show some leadership and sack Mr Lewis. Otherwise your claim that the Conservative Party represents change will prove to be nothing but a shallow public relations exercise.”

A Conservative Party spokesman said: “These are Dr Lewis’ long held and personal views, they are not the view of the Conservative Party and the terms in which he expressed them is wrong. Dr Lewis did, on a free vote, support civil partnerships.

“Under this Labour government we have seen a massive increase in HIV infections and STDs across all the population – straight and gay. Labour has failed to tackle the crisis in sexual health which is why a Conservative government would make it a priority. We would protect spending on public health and do more to give people the information they need to live healthy lives.”

Dr Lewis uses the title of doctor, although he is not a medical professional. Instead, he holds a doctorate from Oxford University

He has voted against most gay rights measures, such as adoption rights for gay parents and the repeal of Section 28. However, he wrote in his letter that he supported civil partnerships.

He wrote: “On the other hand (though no one seems to have noticed), I voted in favour of the Civil Partnerships Bill. One of the criticisms commonly made of gay relationships is that very often they do not last.

“It therefore seems obvious to me that, when a gay couple wish to commit to each other, by forming a permanent relationships, they should be encouraged and assisted in every way.”

Shadow home secretary Chris Grayling recently had to apologise after he was secretly filmed telling a meeting that he agreed bed and breakfast owners should have the right to bar gay couples.