The UK Border Agency has rejected a call by prison inspectors to stop using force on pregnant women and children it is trying to remove from the UK, according to an internal government document seen by the Guardian.

The document contains UKBA's response to recommendations for improvement at the government's new child detention facility, Cedars, near Gatwick airport, by Her Majesty's Inspectorate of Prisons.

Inspectors said force should never be used to effect the removal of pregnant women or children but UKBA has written "reject" alongside this recommendation, saying that without it removals could be delayed, leading families to strengthen their ties with the UK.

They add that physical intervention could also not be ruled out with children and that "the adoption of new techniques for use on children are under consideration".

The HMIP report contained much praise for Cedars, describing it as "an exceptional facility [which] has many practices which should be replicated in other areas of detention." However, there are criticisms too, particularly about force used against pregnant women and children. The inspectors highlighted a particular incident in which a pregnant woman had her wheelchair tipped up and her feet held by G4S when she resisted the "substantial force" they applied to her.

However the inspectors added that the woman's husband had been disruptive the night before the planned removal, he had been pushing and kicking doors and causing some damage. Inspectors found that force had been used on six of the 39 families who had been in the detention facility by the time of the inspection.

The Guardian has obtained a 17-page, handwritten complaint from the pregnant woman in the wheelchair about her treatment prior to and during removal. She was unable to lodge a complaint before she, her husband and young son were forced on to a plane but managed to send the letter after being returned to her home country.

The letter states: "The … woman from G4S pressed my belly. I cried from pain. I said: 'you hurt my belly, you hurt my baby' she refused to stop. They began to drag me from wheelchair to floor, from floor to wheelchair. I was resisting. They were like animals. I was dragged through corridors, I was dragged like a dog."

The woman claimed three or four G4S men in helmets, holding riot shields, jumped on her husband "like my husband was a terrorist" and that he too was treated like a dog. She said her body was covered in bruises after the incident.

G4S said: "In this incident our staff were concerned that the woman risked causing self-harm." The Guardian has also learned of another incident in which a pregnant woman miscarried at the airport shortly before she was due to be put on a plane. It is understood that force was not used against her although she was separated from her husband during part of her stay at Cedars, which caused her great distress. It is understood that both pregnant women reported that they were bleeding hours before they were due to be removed. In the case of the woman in the wheelchair an independent doctor warned that putting the woman on the plane without adequate monitoring while she was bleeding could lead to premature labour and ruptured membranes.

HMIP made 27 recommendations for improvement after an inspection little more than six months after Cedars opened in September 2011. The UKBA response document, referred to as a service improvement plan, received ministerial backing on 21 December last year.

UKBA has rejected four of the recommendations. Officials are refusing to delay the removal of detainees who allege assault during removal when the assault is backed by medical evidence. They say complaints can be investigated after the detainee has been deported.

In practice such investigations are likely to be extremely difficult to conduct as some detainees are arrested and detained on return home. Others go underground and are unable to maintain phone or email contact with the UK.

Medical Justice, a charity supporting people in immigration detention and investigates health abuses there, is conducting research into the treatment of pregnant women detainees.

Its co-ordinator, Emma Mlotshwa, said: "UKBA has sanctioned violence against children, pregnant women and their unborn babies. Though they have outsourced this abuse to their private contractors G4S and Reliance, UKBA are ultimately responsible for it.."

The Refugee Children's Consortium said: "We are appalled that the Border Agency continues to use physical force against children and pregnant women when removing them from the UK, despite the grave concerns raised by the Prisons Inspectorate.

"It is clear that current safeguards are not working. Our members have received disturbing reports of force being used against parents and children by G4S and Reliance staff in the course of the family returns process. It is very troubling that the Border Agency has failed to produce a published policy on the use of force against children, despite this new detention facility for families having opened well over a year ago."

An HMIP spokeswoman said: "Where recommendations are not accepted and the facts have not changed, we will repeat the inspection until progress has been made. We have made it clear that, in our view, even a minimal level of force can quickly escalate and so force should never be used against vulnerable groups to gain compliance but only to prevent harm."

A UKBA spokesperson said: "Where we and the courts have found a family is not in genuine need of asylum we expect them to return home voluntarily. We do everything possible to help them go home including offering assistance once they are back in their home country.

Where a family refuses to accept that support, our last resort is an enforced return. We are committed to treating everyone in our care with dignity and respect and we take advice from our independent family returns panel on every case."