The only witness to an alleged case of sexual misconduct at Yarl's Wood immigration removal centre is to be deported, without being given the opportunity to testify to police.

Sirah Jeng was detained on Thursday inside the centre, hours before a scheduled interview with police. Jeng alleges she witnessed a male Serco employee having sexual contact with a detainee.

She has been given five days' notice of deportation, the minimum period required, and is to be sent back to the Gambia on Tuesday. Campaigners accuse the Home Office of attempting to "silence" Jeng by removing her from the UK before she can be questioned by officers.

The 59-year-old told lawyers that she saw a Serco employee kissing a 23-year-old detainee called Tanja last year in Yarl's Wood. CCTV footage corroborates her testimony. Jeng is thought to be the only eyewitness to the alleged sexual contact and is therefore crucial to an ongoing investigation by Bedfordshire police into abuse allegations at Britain's largest immigration removal centre for women. Three Serco staff have been dismissed over Tanja's allegations, which her lawyer, Harriet Wistrich, believes amount to misconduct in public office.

Jeng, who has lived in the UK for 12 years and is married to a British citizen, said: "I saw them kissing. Now they are trying to get rid of me before I can speak to the police."

In August, Bedfordshire police launched an investigation into Tanja's claims that she was subjected to inappropriate sexual behaviour by several guards. Tanja, not her real name, claimed attempts were made to deport her within days of her informing Yarl's Wood's management of the incidents.

Jeng said her police interview was scheduled for 5pm last Thursday. At 11am that day she went to Dallas Court, Manchester's immigration reporting centre for asylum seekers, where she was detained and taken to Yarl's Wood.

Anthony Gard of civil rights group Movement for Justice said: "We are determined that this crude attempt to silence her doesn't succeed. Sirah is so committed to exposing the sexual abuse she witnessed – she knows that this case speaks for many others. A system that abuses women's human rights and dignity as Yarl's Wood does inevitably leads to sexual abuse. When the UKBA interferes with a police inquiry to get a witness out of the country it's a sign of their desperation and the impact the Yarl's Wood women and their allies is having; it means we can defeat the cover-up."

A Home Office spokesman said they could not comment on individual cases.

Last week the chief inspector of prisons said that Yarl's Wood needed to recruit more female staff "as a matter of urgency" following the Tanja incident.

His comments follow an unannounced inspection by HMIP, triggered by allegations in the Observer, which found that female detainees told inspectors they felt uncomfortable when male officers came into their rooms, often without waiting for a reply after knocking. In one instance in September, it is alleged, three UK Border Agency officials entered a room where a 34-year-old woman from Malawi said she was naked in bed.

The woman said: "They came inside and shut the door. One stood at the foot of my bed, another pulled up a chair and sat next to my bed and another stood at the door inside, they were threatening me with removal directions. I was very scared."

Almost nine in 10 of those held at Yarl's Wood are women, yet around half of the staff are male.

A Home Office spokesman said: "We take the welfare of our detainees very seriously and any allegations of misconduct are thoroughly investigated. Detention and removal are essential parts of effective immigration controls, but it is vital that these are carried out with dignity and respect. We also operate a comprehensive complaints system for detainees who feel that they have not been treated in accordance with our published operating standards."

Bedforshire police said they could not talk about witnesses or case details, but that the case was ongoing.