A conservative group has said that allowing gays to serve openly in the US military could threaten national security.

A letter urging President Barack Obama to preserve the gay ban was leaked to the Palm Center, a research institute at the University of California.

Written by two retired, four-star Army generals, James Lindsay, 76, and Carl Stiner, 72, it claims that the law must be preserved in order to ensure “good order, discipline and morale” in the military.

They claimed repealing the law would undermine recruitment and retention of staff, impact on leadership, dissuade parents from allowing their sons and daughters to sign up and ultimately be a danger to national security.

Last summer, Elaine Donnelly, president of the Center for Military Readiness, came under national criticism when she told congress that gays would “sexualise the atmosphere” and engage in “passive aggressive behavior.”

However, experts have said that the campaign may actually lead to the kind of disruptions gays are accused of causing.

“The most important factor in lifting a gay ban is a clear signal from senior leadership,” said Dr. Nathaniel Frank, a senior research fellow at the Palm Center and author of “Unfriendly Fire: How the Gay Ban Undermines the Military and Weakens America.

“Everyone knows it’s just a matter of time before the gay ban falls, so for officers to come out and say ‘gays are a threat to the military’ could cause the very problems that they ostensibly fear.”

US Defence Secretary Robert Gates said this week that the military’s ‘Don’t Ask, Don’t Tell’ policy regarding gay and lesbian service members is unlikely to be changed any time soon.

Speaking on Fox News Sunday, he said: “The president and I feel like we’ve got a lot on our plates right now and let’s push that one down the road a little bit.”

“It continues to be the law and any change in policy would require a change in the law,” Gates said. “We will follow the law, whatever it is.

“That dialogue, though, has really not progressed very far at this point in the administration,” he added.

In January, Obama’s press secretary said the administration was planning to end the gay ban.

Responding to a question on whether the administration would repeal the policy, he said: “You don’t hear a politician give a one-word answer much, but it’s ‘Yes.’”