Gay and lesbian prisoners in California are to be allowed conjugal visits from their partners, under a new prison policy revealed on Friday.

The change in policy came after gay and civil rights groups threatened to sue the prisons under equality laws.

The American Civil Liberties Union (ACLU) sent a letter threatening legal action on behalf of Vernon Foeller, 40, a gay inmate serving a 20-month sentence for attempted burglary.

Alex Cleghorn, an ACLU lawyers, told The New York Times that Mr Foeller was legally eligible for an overnight stay with a family member, but the prison system “didn’t recognise his partner as a family member.”

Mr Cleghorn said: “They have pages and pages of regulations that must be met to permit these visits, and Vernon met all of these requirements.”

Registered domestic partners, including same-sex couples, were granted full equal rights with married couples in California in 2003.

Mr Foeller was released in April.

“It’s a little troubling that a state agency had to be threatened with legal action to obey state law,” said Geoff Kors, the executive director of Equality California, a gay rights organisation. “There was no justifiable excuse for not complying.”

But Terry Thornton, a spokesperson for the state Department of Corrections and Rehabilitation, said that the delay in change of policy was due to concern about exposing gay prisoners to danger.

She said: “We had to thoroughly evaluate all the security concerns.”

Only gay couples registered as domestic partners will be allowed the conjugal visits, but there is no record of how many domestic partners are serving prison terms.

Very few states in America offer conjugal visits at all. Prison officials say overnight visits from family can help reduce the stress of prison life, and that it can strengthen family bonds.

But critics say it is too expensive to keep separate grounds for conjugal visits, and that they can help the spread of sexually transmitted diseases.

Socialite heiress Paris Hilton is currently serving a prison sentence for violating parole in Los Angeles County Jail, California.

British prisons do not allow conjugal visits for heterosexual or gay couples.