Priti Patel has failed to disclose key documents to Home Office staff detailing how they should decide which migrants might be eligible to be removed to Rwanda, the leader of the UK’s biggest civil service union has claimed. Mark Serwotka, the general secretary of the Public and Commercial Services (PCS) union which represents departmental staff and Border Force officers, said the home secretary’s decision to withhold framework documents explaining the government’s offshoring policy raised suspicions that it was “built on sand” and a cynical attempt to score “cheap political points”. In an article for the Guardian, Serwotka pledged to back legal challenges against the policy on behalf of the Home Office staff who would be asked to deal with its “disastrous consequences”. It comes amid growing suspicion that the policy, announced last month amid the Partygate scandal, was ill-thought through and was expected by the government to be stalled in the courts. On Wednesday, Boris Johnson attempted to blame “liberal lawyers” for delays in the implementation of the policy. Serwotka wrote that the government was asked last month to provide documents outlining the roles of union members who would be expected to implement the policy, but none had been disclosed. He said it followed a similar failure last year to fully disclose documents related to its failed pushback policy. “Our negotiators in the Home Office have requested policy and operational documents but none have been forthcoming. We were forced to follow a similar process in the pushbacks case where those documents that were provided were heavily redacted,” he wrote. “It can only be concluded that either the Rwanda policy announcements are built on sand and no policy framework exists to facilitate it, or they have something to hide.” The government had pledged not to send anyone to Rwanda before responding to the union’s request, Serwotka said. “We have made it clear to the government that they are required to exercise its discretionary powers transparently. In another important step, we have been given an assurance by the home secretary that no action will be taken to remove any individual refugee to Rwanda before we receive a response to our letter. We will seek to extend that assurance until the legality of the proposals has been tested,” he wrote. Patel announced the policy last month after pledging a payment of £120m to the Rwandan government – a payment which does not cover flights, security or living costs. Under the plans, men and women who arrived in the UK via irregular routes would be processed in the UK. If not eligible for asylum, they could then be sent 4,500 miles to Kigali where they would be encouraged to settle in housing paid for by the UK. They would have no right to return to the UK and there was no formal appeals procedure to the UK’s decision. Last week, the Guardian disclosed that the PCS was one of four organisations challenging the policy in the courts. Johnson had said he wanted to send migrants to the authoritarian east African country before the end of May, but his spokesperson on Tuesday told reporters the programme would start “within months”. Speaking at Southampton airport, Johnson told reporters on Wednesday that the plans would be challenged in the courts, but defended the implementation. “Of course, there are going to be legal eagles, liberal lawyers, who will try to make this difficult to settle. We always knew this was going to happen, but it is a very, very sensible thing,” he said. Serwotka wrote that after being part of a successful legal challenge against the government’s pushback policy, the union was backing a similar challenge against offshoring. “As we did in the pushbacks case, this union intends to do all it can to stop our diligent public servants from being placed in this unjust position once more,” he wrote. An analysis of government figures on Wednesday showed that 7,240 people had reached the UK in small boats from France in the four months since the start of 2022. A Home Office spokesperson said the partnership with Rwanda “means those arriving dangerously, illegally or unnecessarily can be relocated to have their asylum claims considered and, if recognised as refugees, build their lives there”, adding: “This partnership fully complies with international and national law, and we will defend any legal challenge robustly.”