Officials from Bahrain’s prison torture inspection panel were allowed secretly to visit Yarl’s Wood, Britain’s most controversial immigration detention centre for women, raising fresh questions over the nature of the relationship between the UK and the Gulf state.

In June 2013, delegates from Bahrain, where allegations of torture in police custody and in prisons are widespread, were given permission to access the centre in Bedfordshire accompanied by members of the UK prison watchdog.

The visit is fresh evidence of the growing and often secret relationship between security officials in the two countries that has developed despite repeated allegations of torture in Bahraini detention centres.

And it follows repeated claims of ill treatment at Yarl’s Wood. It also came shortly before an official UN inspector investigating state-run detention centres for women was banned by the Home Office from entering Yarl’s Wood, which houses about 400 women.

On Friday, it emerged that a controversial multimillion-pound programme of support for Bahrain’s security and justice system was being bolstered by a further £2m of British funding, despite the Gulf state reversing reforms to an intelligence agency accused of torture.

On Saturday the families of three men on death row expressed fears that their executions were imminent, which would make them the first in the Gulf kingdom since 2010. Officials from Bahrain’s death-row prison called the men’s families to tell them to visit the prison but refused to explain why. Campaigners noted that several unofficial pro-government Twitter accounts had tweeted about possible executions.

Facebook Twitter Pinterest Death-row prisoner Mohammed Ramadan, pictured in prison in Bahrain with his son, was allegedly tortured. Photograph: Via Reprieve

Meanwhile, documents obtained by the legal charity Reprieve reveal that officials from Her Majesty’s Inspectorate of Prisons visited Bahrain and helped prepare for an official inspection of the country’s most notorious police station, where a number of inmates have alleged ill treatment.

The inspection of Bahrain’s CID building took place on 24-25 December 2014, leading to a report that ran to only six pages and failed to mention several high-profile allegations of torture.

Now it has emerged that HMIP staff visited Bahrain a fortnight before the inspection, from 7-9 December, “to help PDRC [Prisoners and Detainees Rights Commission] do detailed planning for police custody inspections, finalise a police custody detainees interview questionnaire”. Reprieve said it was concerned by the failure of the subsequent report to mention high-profile allegations of torture, citing the experiences of a death-row prisoner, Mohammed Ramadan, a father of three. He was tortured for four days at the CID building in March 2014, ultimately making a false confession that was used as the basis of his death sentence. The PDRC inspection report does not mention the allegations despite Ramadan’s jailers being accused of beating him.

British lawyers are also aware of three other death-row inmates who were sentenced to death on the basis of false confessions made under torture at the CID building in March 2014.

Maya Foa, a director of Reprieve, said: “It is scandalous to discover that the HMIP helped plan a police custody inspection which failed to address well-known torture cases at Bahrain’s most brutal police station. Five men have been sentenced to death on the basis of false confessions extracted under torture in the CID building. Many of these allegations were public at the time of this inspection, so why on earth did the report fail to even mention them?”

An HMIP spokeswoman said the tour of Yarl’s Wood was designed to help their Bahrain counterparts become better inspectors. She said: “They accompanied us on some inspections in England so that they could see how we conduct independent investigations and in particular how we listen in great detail to what we are told by detainees.”

She said HMIP had encouraged the development of an inspection mechanism in Bahrain to help prevent torture, adding they had not conducted any inspections there and had made no financial gain. “We are well aware that there have been allegations of serious abuse in all forms of custody in Bahrain, including police custody, and have urged the PDRC to focus on three key priorities in inspections and reports: examining the response to allegations of torture; including the detainee voice in all reports; and producing well structured reports with clear judgments.

“We have, in private, provided detailed feedback to the PDRC on their work and will continue to support them as long as they maintain a commitment to progress. We keep our decision on future involvement in the project under review on this basis. We have done no recent work with the PDRC.”