G4S has launched an internal investigation after a judge referred a number of its employees for prosecution for forgery and contempt of court in a "truly shocking" case of what he called disgraceful behaviour.

In the high court, Mr Justice Mostyn said three employees from G4S running Brook House immigration removal centre in Gatwick, East Sussex, had been involved in forging a document and contempt of court after giving witness statements during an immigration appeal involving allegations of torture at the hands of a foreign government.

In an excoriating judgment which has been referred the attorney general and the Director of Public Prosecutions (DPP), Mostyn said G4S employees Tamara Burns, Marilyn Bennett and Matthew Newman were involved in "corruptly redacting" an official certificate, an action which helped bolster the case against an immigrant who was being deported from the UK.

Last week the Guardian revealed the Home Office is in discussions with G4S to expand Brook House by 30% despite a freeze on new government contracts for the multinational security company while it is investigated by the Serious Fraud Office for allegations of criminally overcharging taxpayers.

T​he asylum seeker, who is not allowed to be named for legal reasons, claimed he had been tortured and beaten with a heated metal rod on arrival at his country of origin after UK officials refused to remove paperwork from his luggage which identified him with anti-government organisations there.

​Mostyn found that after his room at Brook House was cleared of belongings, a certificate was drawn up by officials making reference to "various paperwork".

But during court proceedings, centre staff submitted a second "doctored" certificate, which scrubbed all mention of the paperwork.

"The conduct of the secretary of state's agents in falsifying the room clearance certificate is corrupt and truly shocking," Justice Mostyn said in his judgment Thursday.

"The original clean certificate was only produced by the secretary of state following a request made by the claimant's solicitors after the doctored certificate has been produced in evidence as exhibits to the witness statements of Tamara Burns, Marilyn Bennett and Matthew Newman," said ​Mostyn.

The three officials behaved disgracefully, the judge said, adding: "When agents of the state falsify documents it undermines, if not fatally then certainly very seriously, the trust of the people in the operation of the rule of law.

"It makes no difference if, as here, the agents are private contractors to whom the secretary of state has outsourced her powers. Corruption by state officials is insidious and corrosive and it is the duty of the authorities where it is found to root it out ruthlessly."

Mostyn referred his findings of forgery and providing false witness testimony to the DPP and the attorney general. However, he added that his "finding as to the turpitude of the agents of the secretary of state" did not detract from the "lack of credibility of the claimant", and he dismissed the appeal.

G4S said one employee was no longer with the company and the remaining two had been suspended with immediate effect. "The allegations made in respect of the conduct of our employees are extremely serious, and we have launched an immediate internal investigation," a G4S official said.

"Due to the nature of these allegations, we have suspended the two personnel concerned with immediate effect, pending the outcome of the investigation.

"We will, of course, co-operate fully with any inquiries the appropriate authorities may pursue in connection with this case."