Rishi Sunak considered banning thousands of workers from joining a union, according to leaked government emails detailing proposals described as potentially the “biggest attack on workers’ rights and freedoms” for decades. The messages, shared between senior civil servants and seen by the Observer, reveal that the prime minister contemplated banning Border Force (BF) staff from trade union membership under its anti-strike legislation announced last Thursday. Union leaders fear the extreme measures – not even known to be under consideration until now – could have also been considered for other sectors, theoretically leading to more than a million workers banned from joining unions. Mark Serwotka, general secretary of the Public and Commercial Services Union, said: “These emails reveal that while the government publicly is saying: ‘We want to resolve the dispute’, behind the scenes they were preparing the biggest attacks on fundamental rights and freedoms that we would have seen in this country for generations.” The emails, drawn up by officials and lawyers in the Department for Business, Energy and Industrial Strategy (BEIS) and shared last month with senior civil servants, presented three models for Sunak to consider to form the government’s anti-strike laws. The first, described as a “police service ban on striking” because officers are banned from industrial action, advocated “BF staff banned from joining a trade union” with striking or “inciting disaffection” to become a criminal offence. Another model, a “prison service-style ban on striking”, would replicate restrictions on prison officers who are also banned from industrial action with possible concessions such as a new independent pay review body. The third option – the one chosen last week – was legislation to enforce “minimum service levels” in public sectors such as the NHS, with employers able to sue unions and sack staff if minimum standards are not met. All three were put to Downing Street last month with the emails revealing that “we [senior BEIS officials] do not yet have a firm view on the preferred model from PM”. The emails, however, show that the most extreme model – banning workers from trade union membership – was rejected only because it might “be difficult to justify” because the European Convention on Human Rights guaranteed UK workers the right to join a union. Because of this, civil servants felt the minimum service levels model was their “preferred option”. Serwotka said the proposals suggested this government was more hardline than that of Margaret Thatcher, who provoked uproar after imposing a ban on union membership of employees at GCHQ, the UK’s intelligence, security and cyber agency. “This takes Thatcher’s doctrine to whole new levels,” said Serwotka, whose union is the largest representing civil servants in the UK, with more than 100,000 members, including 4,000 Border Force staff. Although the leaked emails cite only Border Force staff as being banned from joining a union, PCS sources believe the restrictions would have been applied to other sectors. The government’s minimum service levels legislation affects six sectors including border security, health, education, fire, ambulance, rail and nuclear commissioning, and Serwotka believes that the government would have attempted to ban union membership to workers in all these areas. “We know that the legislation covers health service, teachers and transport, and one can assume they would have considered this option not just for the Border Force but everywhere. “That would affect over a million people, which is an extraordinary step in any democratic society … They’re trying to potentially take what are already the most restrictive anti-union laws in Europe and take them to levels I don’t think anybody thought they would seriously contemplate.” The emails also show that civil servants accept that even the minimum service option is vulnerable to legal challenges. They admit that the legislation is “not without its challenges” and that the “real test” will come when challenges are made under article 11 of the ECHR, which protects a person’s right to protest. A government spokesperson said: “As announced this week, we are introducing new laws to ensure a minimum level of safety in some of our most crucial sectors when industrial action takes place.”