Boris Johnson has responded to the biggest rail strikes in a generation with plans to break the industrial action by allowing firms to bring in agency staff, a move unions have decried as unworkable, unsafe and potentially breaking international law.

As 40,000 workers prepared for Tuesday’s strike, the most wide-reaching on the railways in 30 years, Downing Street brought forward changes to enable employers to replace employees with temporary staff.

The highly controversial measure would make disputes long and bitter, unions warned on Monday, with the Trades Union Congress (TUC) accusing Johnson of taking a step that “even Margaret Thatcher did not go near”.

Instead, it would inflame divisions between employers and trade unions when the government should be trying to bring about a deal, they said.

However, the prime minister accused the union of “too high demands on pay” and said it was necessary for public-sector employers to show “pay discipline and restraint” to prevent even higher inflation, saying it was necessary salary deals remain “sensible”.

Earlier this year, Downing Street had disagreed with the Bank of England governor, Andrew Bailey, when he said workers should not expect pay rises matching inflation, saying Johnson wanted a “high-wage economy”. But on Monday the prime minister stressed the need for public-sector pay to be kept down, with No 10 saying the government had a “responsibility to tackle inflation and stop it becoming entrenched”.

The rail strikes are due to cause the cancellation of about 80% of train services on Tuesday, with further action scheduled for Thursday and Saturday, after talks between rail operators and the RMT union broke down. London Underground workers will also walk out for 24 hours on Tuesday.

Mick Lynch, the RMT’s general secretary, raised the prospect of further strikes throughout the summer, as the two sides remained far apart and ministers refused to join the negotiating table.

Lynch said the offers were unacceptable. “What we’ve come to understand is the dead hand of this Tory government is all over this dispute – and the fingerprints of Grant Shapps, the transport secretary, and the DNA of Rishi Sunak, the chancellor, are all over the problems in the railway, and indeed in this society.”

He said the source of the dispute was the government’s decision to “slash £4bn of funding from national rail and TfL … forcing companies to implement transport austerity … and they have prevented a settlement to this dispute”.

On BBC’s Newsnight on Monday night, Lynch said Network Rail had “escalated” the dispute at the talks that day by telling him there would be redundancies from 1 July. It came during a heated debate with digital minister, Chris Philp, in which he frequently accused the Conservative MP of being a liar.

Union leaders and a leading recruitment body warned on Monday that the government’s plans to repeal the ban on strike-breaking would only make things worse. No official announcement has been made, but Kwasi Kwarteng, the business secretary, tweeted: “Repealing these 1970s-era restrictions will give businesses the freedom to access skilled, temporary staff at short notice. Legislation is on its way.”

Whitehall sources said the drive for the new anti-strike laws was, however, coming from No 10 and the Cabinet Office for political reasons, rather than the business department.

Paul Nowak, the deputy general secretary of the TUC, said: “Laws against bringing in agency workers have been in place since this was outlawed in 1973. Even Margaret Thatcher didn’t go near it. But Boris Johnson has pulled out the playbook.” He said the prime minister appeared to be trying to unite his own side around a conflict with trade unions as “part of Operation Save Big Dog” – the nickname for the effort to shore up his flagging premiership.

Nowak said there were safety concerns with bringing in agency workers, who might have little experience in what they are being asked to do, and would be put in an “uncomfortable position” of having to cross a picket line.

“It prolongs disputes. It makes them very bitter. The use of agency workers themselves becomes another point of conflict between employers and unions,” he said. “We have real concerns agency workers will be pitted against directly employed staff.”

He also questioned the legality of repealing the ban. “Once again, this government is showing its disregard for international law, which these proposals almost certainly breach,” Nowak added, citing the right to take strike action under the principles of the UN’s International Labour Organisation.

A joint statement from the TUC and the Recruitment and Employment Federation (REC) said the plan was counterproductive, impractical and would put workers at risk.

Neil Carberry, the REC’s chief executive, said: “The government’s proposal will not work. Agency staff have a choice of roles and are highly unlikely to choose to cross picket lines.”

The plans would affect not just the railways, but many other sectors where unions are considering strike ballots, including NHS staff, teachers, care workers, civil servants and refuse collectors.

Unions representing NHS staff also criticised the government’s plan to encourage the use of agency workers in hospitals and other healthcare settings as unworkable and a threat to patients’ safety.

Joanne Galbraith-Marten, the Royal College of Nursing’s director of employment relations and legal services, said: “This change would be undemocratic and unsafe. Any industrial action by our members is very carefully planned to keep patients safe. Bringing in less qualified or agency workers could put patients at risk.”

The Managers in Partnership union, which represents NHS managers, said the government was “barking up the wrong tree” by proposing the temporary replacement of striking NHS staff.

“Health unions normally guarantee ‘life and limb’, cover so they don’t call all their members to strike, and therefore the government risks being alarmist,” said Jon Restell, its chief executive.

“There will be a raft of healthcare regulatory constraints on clinical staffing and service delivery. The government would be pretty reckless if it tried to ditch those for a period of industrial action,” he said.