Home Office officials are under fire for using "shockingly degrading" lines of questioning when interviewing gay and lesbian asylum seekers, despite clear guidance that such an approach is unacceptable.

A confidential Home Office document leaked to the Observer reveals how one bisexual asylum seeker was asked a series of lurid questions by a Home Office official, including: "Did you put your penis into x's backside?" and "When x was penetrating you, did you have an erection? Did x ejaculate inside you. Why did you use a condom?"

The document reveals that during five hours of questioning in a UK detention centre, the male asylum seeker was also asked: "What is it about men's backsides that attracts you?" and "What is it about the way men walk that turns you on?"

The questions, typed up by a Home Office employee, and dated last October, have been branded an "interrogation".

The Home Office on Saturday admitted that staff were "not permitted to ask inappropriate or intrusive questions", but added that attempts to determine an individual's sexual orientation were conducted "as sensitively as possible".

S Chelvan, a barrister and expert in asylum claims based on sexuality, said the interview, which was conducted with no lawyer present, was "shockingly degrading".

He added: "I'm horrified by the nature of the questions that have been highlighted. It's more like an interrogation than an interview. It is exceptionally troubling that there were questions like whether an individual ejaculated or whether they used a condom. This is an unacceptable investigation of a gay asylum claim. Clearly, something is going terribly wrong here."

Immigration barrister Colin Yeo also voiced concern: "This is the worst I have seen, but these sorts of intrusive, abusive questions are features of Home Office interview practice, particularly in cases involving sexuality. The underlying problem is that officials believe everyone is a liar. It leads to a fundamental lack of respect for the people they are dealing with."

Keith Vaz, chair of the home affairs select committee, said he was "shocked" that Home Office officials were still questioning asylum seekers in such a manner, and called for the practice to be stopped.

Campaigners said it exposed the culture of disbelief of vulnerable asylum seekers. In 2010, a whistle-blower who had worked at a centre for processing asylum seekers' claims revealed how colleagues expressed vehemently anti-immigration views and took pride in refusing applications.

Gay rights group Stonewall, whose studies found almost "systemic homophobia" in the UK's asylum system, said the approach could be "deeply distressing" to asylum seekers.

Richard Lane, spokesman for Stonewall, said: "Valuable time and resources is spent attempting to 'prove' a claimant is gay rather than establishing whether they have a legitimate fear of persecution."

A Home Office spokesperson said: "The UK has a proud history of granting asylum to those who need it and we do not deport anyone at risk of persecution because of their sexuality.

"All applicants are required to establish they face persecution, inhumane or degrading treatment in their home country to qualify for our protection."