A former detainee at Yarl's Wood immigration removal centre has alleged that women held there have been subjected to unwanted sexual advances and abuse by security guards and other officials.

Testimony seen by the Observer and now with police, "Tanja", a 23-year-old Roma woman released from Yarl's Wood last March, describes having had sexual contact with three male guards. Tanja – not her real name – said attempts were made to deport her within days of her informing Yarl's Wood's management of the incidents. She also claims one security guard had inappropriate relations with at least four women.

The claims raise fresh questions over the treatment of vulnerable women at the Bedfordshire site, which is Britain's largest immigration removal centre for women and can house up to 400 people. Sources at Yarl's Wood say that more cases are likely to come to light following Tanja's testimony, as women have been too fearful to come forward until now.

Two alleged victims had left the detention centre by the time the claims were examined and have not been traced. The professional standards unit at the Home Office has advised the UK Border Agency to consider interviewing other women at Yarl's Wood to establish if inappropriate relationships are an endemic problem.

Harriet Wistrich from law firm Birnberg Peirce, who is representing some of the alleged victims, said: "The women in Yarl's Wood are some of the most vulnerable you can imagine. Many have escaped horrific abuse in their own countries; most are very isolated from friends and family. The state has a duty to investigate such serious allegations, but it has repeatedly failed. Now the government wants to remove legal aid altogether for detainees and foreign nationals, giving a green light to abuse at Yarl's Wood to continue." She said there was evidence of "systemic supervisory defects" in the management of the centre.

Yarl's Wood is run by Serco under contract from the Home Office. Earlier this year the company paid an undisclosed sum to a 29-year-old asylum seeker from Pakistan who claimed she was sexually assaulted by a nurse at Yarl's Wood, although the company did not admit liability. But the shocking detail of the latest allegations is likely to trigger a full-blown inquiry into the running of the centre.

Tanja said: "A lot of officers were taking advantage of the girls that were detained. They would promise favours or offer to make life easier, saying they would have more chance of winning their case or staying in the country." In a formal witness statement she has sent to Bedfordshire police, she states that one Serco official she was involved with sexually told her: "Don't worry, there is no way they can deport you."

She also claimed the sexual contact was not all consensual. Referring to one occasion, she stated: "I said I was scared and I did not want to ... There were two occasions when I was made to do 'blow jobs' when I did not want to. [The guard] was well aware that I did not want to."

Wistrich said questions needed to be asked over whether consent could ever be freely given by individuals who were in such a vulnerable situation.

A letter to the UK Border Agency from Wistrich concerning Tanja states: "Our client has indicated that she was a reluctant participant in some of the sexual contact and, given the huge power imbalance, at least some of the conduct by the officers she complains about may amount to assault. Furthermore, she has suggested that there have been attempts by officers to destroy and suppress evidence of their misconduct."

A letter to Tanja from the UK Border Agency said it had found "sufficient evidence" to indicate that Serco guards "did behave in an unprofessional way".

John Tolland, Serco's director at Yarl's Wood, said: "When an allegation of sexually inappropriate behaviour between staff and a resident was brought to our attention in 2012 the matter was fully investigated and as a result three members of staff were dismissed."

The UK Border Agency's professional standards unit, which has responsibility for investigating allegations of misconduct by UKBA officials or private contractors, has said the alleged victim did not indicate to its investigator that her sexual contact with guards was "anything other than consensual".

A Home Office spokesman said: "Detention and removal are essential parts of effective immigration controls, but it is vital that these are carried out with humanity and dignity, and we are committed to ensuring that this is the case. We expect contractors to maintain the highest standards.

"Any allegations of misconduct are thoroughly investigated and, if necessary, they will be reported to the appropriate authorities, including the police. We operate a comprehensive complaints system for detainees who feel they have not been treated in accordance with our published operating standards."