A cross-party group of British MPs has accused the Saudi government of subjecting female activists being held in prison to torture and warned that the detainees may suffer serious long-term health problems unless they receive urgent medical care.

A Detention Review Panel (DRP) of MPs and human rights groups claim in a highly critical report that Saudi officials at “the highest levels” could be held culpable for abuse “meeting the threshold for the crime of torture under both Saudi and international law”, enabling prosecution to take place under the principles of universal jurisdiction.

The MPs hold that the detainees have been subjected to cruel and inhumane treatment including assault, sleep deprivation, threats to life and solitary confinement. They are passing on their report to the office of the UN’s special rapporteur with an official request that the abuse allegations are fully investigated.

The DRP called, in the document, on the Saudi government to end the mistreatment and for the immediate release of the women detainees and an investigation, by the authorities in Riyadh, into the allegations of abuse.

Conservative MP Crispin Blunt, the DRP chair, said: “Our conclusions are stark. The Saudi women activist detainees have been treated so badly as to sustain an international investigation for torture. Denied proper access to medical care, legal advice or visits from their families, their solitary confinement and mistreatment are severe enough to meet the international definition of torture. The supervisory chain of command up to the highest levels of Saudi authority would be responsible for this.”

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Liberal Democrat MP Layla Moran said: “When I heard of the arrests, I was, like many people, shocked that it had happened at all. The torture, in particular allegations of sexual harassment and threats of rape, are inexcusable.”

Labour MP Paul Williams, a member of the Health and Social Care Committee, added: “The allegations of mistreatment faced by the detainees have shocked the international community. We are open to discussing our report with the Saudi authorities and to receiving any evidence they have so we can assess our conclusions on the basis of the fullest information available.”

Tayab Ali, partner at ITN Solicitors, who acts as the DRP legal secretariat and rapporteur, said: “In light of the serious allegations of torture we have heard and the Saudi authorities’ continuing failure to respond to us, my client will now be working with the international community to ensure that the mistreatment of the detainees does not continue.

“My client will be referring the DRP’s findings, along with a complaint to the United Nations to request that the special rapporteur on torture and the Working Group on Arbitrary Detention fully investigate the detention conditions faced by the Saudi women activist detainees.”

The DRP had asked for, and been denied, access to the detainees by the Saudi government. Riyadh was told that their report would be published from accounts compiled by human rights organisations and activists unless the MPs were allowed into the country to carry out their own investigation.

Mr Blunt had faced criticism in the past for backing British arms sales to Saudi Arabia and supporting the war being waged in Yemen by the Saudi-led coalition. He had also declared that Britain was “right to roll out the red carpet” for the visit of the Saudi crown prince, Mohammad bin Salman, last year.

Urging the Saudi authorities to allow access, Mr Blunt said last month: “One can say that it was my support for arms sales to Saudi Arabia that cost me re-election as chair of the Foreign Affairs Committee. The Saudi government should be reassured that there is someone like me on the panel. It should show that they will get a fair hearing.”

Following the report’s publication, Mr Blunt said: “Saudi Arabia stands on the brink. It is not too late to alter course and avert the spiral downwards to catastrophe that the detention of these activists represents.

“We continue to hope that the Saudi authorities see this as an opportunity to recognise and address the contradictions in the treatment of the detainees and claimed aspirations for the future of Saudi Arabia. Vision 2030 will never be delivered if the people who could help deliver it are treated in this appalling manner.”

Vision 2030 was unveiled by the crown prince three years ago as a programme to reduce dependence on oil and diversify the economy while carrying out sweeping modernisation of government policy; it was viewed as a major reforming step.