Three low-paid workers are making an urgent application in the London courts to challenge Boris Johnson’s threat to crash out of the European Union with a no-deal Brexit on 31 October.

All three, who are members of the Independent Workers’ Union of Great Britain (IWGB) and are cleaners or bicycle couriers, are planning to join the civil rights organisation Liberty in a combined effort to ensure the prime minister does not breach the Benn act.

Their action in the English courts is designed to complement a similar claim brought by Scottish campaigners aimed at forcing Johnson to ask for an extension to the UK’s membership of the EU if no agreement is reached by 19 October.

The barrister Jolyon Maugham QC and other activists lost their attempt in Edinburgh for a court order requiring compliance on Monday but will appeal.

The European Union (Withdrawal) Act (No2) 2019, known as the Benn act, mandates the prime minister to write to the EU seeking an extension to article 50 until 31 January in the absence of a deal.

In a letter sent to Downing Street last week, lawyers for Maritza Calle, Wilson Romero, Alexander Marshall and the IWGB said failure to comply with the Benn act “would be self-evidently unlawful”.

Q&A What does a no-deal or WTO-rules Brexit mean? Show Hide If the UK leaves the EU without a deal it would by default, become a “third country”, with no overarching post-Brexit plan in place and no transition period. The UK would no longer be paying into the EU budget, nor would it hand over the £39bn divorce payment. The UK would drop out of countless arrangements, pacts and treaties, covering everything from tariffs to the movement of people, foodstuffs, other goods and data, to numerous specific deals on things such as aviation, and policing and security. Without an overall withdrawal agreement each element would need to be agreed. In the immediate aftermath, without a deal the UK would trade with the EU on the default terms of the World Trade Organization (WTO), including tariffs on agricultural goods. The UK government has already indicated that it will set low or no tariffs on goods coming into the country. This would lower the price of imports – making it harder for British manufacturers to compete with foreign goods. If the UK sets the tariffs to zero on goods coming in from the EU, under WTO “most favoured nation” rules it must also offer the same zero tariffs to other countries.

WTO rules only cover goods – they do not apply to financial services, a significant part of the UK’s economy. Trading under WTO rules will also require border checks, which could cause delays at ports, and a severe challenge to the peace process in Ireland without alternative arrangements in place to avoid a hard border.

Some no-deal supporters have claimed that the UK can use article XXIV of the General Agreement on Tariffs and Trade (Gatt) to force the EU to accept a period of up to 10 years where there are no tariffs while a free trade agreement is negotiated. However, the UK cannot invoke article XXIV unilaterally – the EU would have to agree to it. In previous cases where the article has been used, the two sides had a deal in place, and it has never been used to replicate something of the scale and complexity of the EU and the UK’s trading relationship. The director general of the WTO, Roberto Azevêdo, has told Prospect magazine that “in simple factual terms in this scenario, you could expect to see the application of tariffs between the UK and EU where currently there are none”. Until some agreements are in place, a no-deal scenario will place extra overheads on UK businesses – eg the current government advice is that all drivers, including lorries and commercial vehicles, will require extra documentation to be able to drive in Europeif there is no deal. Those arguing for a “managed” no deal envisage that a range of smaller, sector-by-sector, bilateral agreements could be quickly put into place as mutual self-interest between the UK and EU to avoid introducing or to rapidly remove this kind of bureaucracy. Martin Belam

Calle and Romero are Spanish passport-holders who have worked in the UK since 2012 and 2013 respectively. Both have UK residency rights through EU citizenship. Marshall is a UK national who works as a bicycle courier and is involved in a European court of justice case on holiday rights.

Marshall, the chair of the IWGB’s couriers and logistics branch, said: “Without the EU charter of fundamental rights and the European court it would have been impossible for my colleagues and I to bring the landmark £1m holiday pay claim against our employer, The Doctors Laboratory.

“Crashing out with a no-deal Brexit could put the case at risk and couriers like myself would never see the thousands of pounds they are owed by the company. But more importantly, it would mean that other so-called ‘gig economy’ workers would not be able to rely on EU law to defend their rights to paid holidays and protection from discrimination among other things.”

A no-deal departure would have “serious adverse consequences” for UK workers and the three IWGB members in particular, lawyers for all three claimants said in their letter. “In the absence of any undertaking by [Johnson] that he will comply with his statutory obligations”, the lawyers will seek a declaration and mandatory order.

Johnson’s formal undertaking through government lawyers in the Scottish courts last week promising to seek an extension came despite contradictory comments made outside court in which he has repeatedly implied that the UK will leave on 31 October with or without a deal. The IWGB received a similar written response from Downing Street formally pledging that he would seek an extension.

Dr Jason Moyer-Lee, the general secretary of the IWGB, said: “For years we have been trying to get the government to force companies to obey the law. How can we expect that if the government itself doesn’t obey the law? The rule of law is crucial for low-paid workers.”

Liberty, which has been raising money through the CrowdJustice fundraising website for its claim, said it was launching the action not because it was a Brexit issue but because it opposed the prime minister’s threats to break the law.

“Boris Johnson has said he will ignore this act of parliament,” Liberty said. “This will set an extraordinarily dangerous precedent. This is not a Brexit issue. No one is above the law – and if the prime minister breaks it, we’ll see him in court.

“There is much confusion about whether the government will comply with its obligations under the Benn act,” it said. “The prime minister’s lawyers have told us it will obey the law, but Boris Johnson himself has contradicted that claim.” No date has yet been set for any court hearing.

Meanwhile the Shrewsbury Conservative MP and keen Brexiter Daniel Kawczynski is understood to be considering rival legal action to challenge the validity of the Benn act in the courts.

As the legal prospects became more complicated, one academic, Dr Robert Basedow at the London School of Economics’ European Institute, has suggested that the European court of justice in Luxembourg could even become involved in adjudicating whether the UK had actually left the EU legally under article 50 after 31 October.