Home Secretary Priti Patel is facing High Court challenges over controversial plans to send asylum seekers to Rwanda.

A “world-first” agreement with the east African nation, which will see it receive asylum seekers deemed by the UK to have arrived “illegally” and therefore inadmissible under new immigration rules, was signed earlier this month.

The latest step in the Government’s plan to curb Channel crossings and overhaul the asylum system has drawn criticism from across the political spectrum, including from the Archbishop of Canterbury Justin Welby.

The charity Freedom From Torture, which supports survivors of torture who are often asylum seekers and refugees, wrote to the Home Office on Wednesday requesting further information, with a view to bringing a High Court claim over the policy.

The first step towards another challenge, from charities Detention Action and Care4Calais – and the Public and Commercial Services (PCS) union which represents the majority of Border Force staff, was taken on Tuesday when they sent a pre-action letter to the Home Office.

A letter from Freedom From Torture’s legal representatives, Leigh Day, says that, while a memorandum of understanding between the UK and Rwanda has been published, information about the policy has not been made publicly available.

It also says the charity has “serious concerns” about the policy and in due course plans to bring a judicial review.

The charity is asking for any document which sets out the policy of relocating asylum seekers to Rwanda, any guidance or rules provided to Home Office staff or other UK officials, any documents relating to co-operation with Rwanda, and any equality impact or risk assessments carried out in relation to the policy.

The letter requests that any response is sent by 4pm on May 4, and the charity plans to crowdfund to cover legal costs of the case.

Meanwhile, representatives for Detention Action, Care4Calais and the PCS have sent a letter which challenges “the Home Secretary’s failure to disclose the criteria dictating which people seeking asylum will be transferred by force to East Africa and which will remain in the UK”, according to a joint press release.

Those groups are also crowdfunding to cover the costs of the possible legal action and are represented by Duncan Lewis Solicitors.

Freedom From Torture, the PCS and Care4Calais were involved in a challenge due to be heard next week against the Home Secretary over plans to “push back” migrants in boats crossing the Channel, but the planned policy was withdrawn on Sunday.

Sonya Sceats, chief executive of Freedom From Torture, said: “This cruel plan is not only deeply immoral and likely unlawful, it would also deny torture survivors and others access to vital trauma services like those provided by Freedom From Torture.

“Up and down the country, people are beginning to mobilise against this Government’s cynical efforts to stoke fear and xenophobia against refugees.

“This action is one part of a wider fight by the caring public to tell the Government that this is not in our name.”

Bella Sankey, director of Detention Action, said: “This legal action is brought on behalf of all those who are vulnerable to medieval-style banishment for daring to seek sanctuary in the UK, as well as the wider British public who have every right to know what the Home Secretary wants to do in their name.

“We believe that this entire policy is unlawful, both the secrecy surrounding the selection criteria and the whole premise of penalising refugees, by expulsion to Rwanda, for fleeing without papers and permission.

“The Rule of Law is fundamental to our Constitution, and despite this Government’s clear disdain for it, we will hold them to account.”

Clare Moseley, founder of Care4Calais, said: “The Government’s plan to send refugees to Rwanda is a threat to the lives of refugees, the international reputation of the United Kingdom and the finances of British people.

“From the suspiciously sparse detail presented so far, it is already clear the plan holds multiple risks and innocent people will be needlessly and cruelly traumatised to score political points.

“A government’s decisions must be transparent, and ministers must be accountable, so that we know what is being done in our name.

“Our recent success in stopping the proposed pushbacks policy only came after the Home Office was compelled to reveal that they knew the plan was unworkable.

“It is a shame that litigation is required to force the Government to do the right thing.”

Mark Serwotka, PCS general secretary, said: “PCS members in the Home Office do a difficult job.

“We have asked the Home Office for details of what precisely they expect our members to do in respect of this policy and the legal basis for it.

“Nothing has been forthcoming. They are again playing fast and loose with our members’ safety and wellbeing.

“Earlier this week, through our legal action, we forced the Home Secretary into a humiliating climbdown when she had to drop her dangerous and inhumane Pushbacks policy.

“We are again determined to take action to prevent our members being placed in an untenable position.”

In a speech earlier this month outlining the Rwanda agreement, Ms Patel said: “The British people are fair and generous when it comes to helping those in need, but the persistent circumventing of our laws and immigration rules and the reality of a system that is open to gaming and criminal exploitation has eroded public support for Britain’s asylum system and those that genuinely need access to it.

“Putting evil people smugglers out of business is a moral imperative. It requires us to use every tool at our disposal – and also to find new solutions.

“That is why today’s migration and economic development partnership with Rwanda is such a major milestone.

“This agreement fully complies with all international and national law, and as part of this ground-breaking agreement, the UK is making a substantial investment in the economic development of Rwanda.”