Rishi Sunak is under pressure from Tory MPs to accelerate new strike-busting legislation, as Britain faces the biggest wave of industrial action since the tail-end of Margaret Thatcher’s government.

More than 1mn days will be lost to strike action in December — the worst disruption in any month since July 1989 — based on unions’ current plans for action by postal and rail workers, NHS staff and civil servants.

On Tuesday, ministers accused the RMT rail union of “holding the country to ransom” by scheduling walkouts over Christmas, after it rejected a 9 per cent two-year offer from Network Rail. The union has also rejected a separate 8 per cent offer from train operating companies.

But unions have accused ministers of wrecking a potential deal by stopping train companies from raising their pay offers and insisting on changes to working practices.

The Department for Transport declined to comment but has previously said it sets the parameters for negotiations and then leaves the rail to negotiate.

RMT general secretary Mick Lynch said industry negotiators had privately admitted to him that they knew recent offers would be “unacceptable” to unions. Luke Chester, the negotiator for the smaller TSSA union, said the government had “scuppered” a deal.

One rail industry executive said ministers had forced train companies to propose changes to working practices that unions have previously fought bitterly against and were unlikely to accept.

Downing Street on Tuesday insisted it would maintain a hard line on pay offers, warning that meeting union demands would cost billions of pounds and “lock in inflation for an even longer period of time”.

Tory MPs have asked why the government has not moved faster with legislation designed to keep the railways running during strikes — promised three years ago but currently stalled in parliament.

“If the Christmas Eve strike goes ahead, we need to crack on with the anti-strike laws early in the new year,” said one minister.

Boris Johnson, the former prime minister, promised the legislation in the general election of 2019 but the bill was only introduced to parliament in late October. A date for its crucial second reading has still not been set amid concerns it could inflame relations with the rail unions.

If the bill becomes law next year, it would require further negotiations, consultations and secondary legislation before taking effect.

“Liz Truss said she’d have this done within 30 days of becoming prime minister. We are now at a point well past that,” said Chris Loder, a Tory member of the transport select committee. “There are questions about the deliverability of this legislation but if the government is going to do this they need to crack on.”

The confrontation between ministers and unions is rapidly turning into one of the biggest tests of the new government, threatening to worsen existing strains on public services and deepen the UK’s recession.

On Tuesday, unions representing more than 26,000 ambulance workers set dates for walkouts on December 21 and 28 that will lengthen delays in answering call-outs, although crews will still provide “life and limb” cover. These will follow walkouts at hospitals across England, Wales and Northern Ireland by up to 100,000 members of the Royal College of Nursing on 15 and 20 December.

Meanwhile the RMT rail union’s 40,000 members will strike on four days in the run-up to Christmas, as well as over the holiday period, with the CWU’s 115,000 members in Royal Mail set to walk out on seven days in December.

There is also anger over pay at the heart of government, with the PCS civil servants union winning a mandate for strikes at public bodies ranging from the Cabinet Office to the British Museum, the UK Space Agency and the Office for National Statistics.

The PCS has already planned strikes at certain courts, jobcentres and working at the Highways Agency in December, and is set to announce dates by members at the Home Office on Wednesday.

As data collected by Strike Map, a campaign group that has mapped 7,000 strikes this year, illustrates, these disputes pose the worst disruption to the economy since the summer of 1989, when simultaneous walkouts by rail, port and local authority staff and workers across large swaths of industry, caused 2.4mn days to be lost to strike action.

But Gary Smith, general secretary at the GMB union, argued that the current wave of unrest had a different dynamic, with union membership now mainly female and in the public sector. He added that ministers’ attempts to portray the strikes as the work of radicals were misleading.

“This is just about ordinary working people across the country, it’s a grassroots rebellion, bubbling and building up everywhere,” he said. “I’ve never seen this level of anger and preparedness of people to get organised.”

Although unions no longer have the power to shut down industry, the wave of action is now affecting the UK economy. Many businesses are feeling the knock-on effects of train cancellations, postal delays and disruption to other key services.

An ONS survey found that one business in eight was affected by strikes in October. Business groups have appealed to the Royal Mail and CWU to resolve their pay dispute, with small traders and online retailers dreading a postal logjam in the holiday trading period. Kate Nicholls, chief executive of UKHospitality has warned that further rail strikes will be “hugely damaging” for the sector’s “golden month of trade” around Christmas.

The damage to the economy will become more significant if junior doctors and teachers walk out in the new year. Scottish teachers have already held a one-day walkout and around half a million teachers across England are voting in ballots that could shut down schools from February.

Paul Dales, at the consultancy Capital Economics, said the strikes under way already directly affected only a small proportion of economic activity but there would be further knock-on effects if school closures and rail strikes stopped parents and commuters working. “Any hit to GDP is not helpful when the economy is probably already in recession,” he said.

Despite the disruption, public sympathy for striking public sector workers is strengthening as double-digit inflation eats into wages and drives households into real hardship. According to a YouGov poll published last week, people have become more likely since June to support nurses’, teachers’, doctors’ and train drivers’ right to strike.

However, opinion is polarised. Even as support for public-sector workers has grown, the proportion of people who think trade unions play a negative role has also increased, with a third of those polled backing tougher restrictions on their ability to take action.

If the government does push through the transport strikes legislation in the new year, it may not take effect until late in 2023. However, senior government figures are considering a further tightening of the rules governing strikes.

One way to do this would be to lift the threshold for the percentage of members voting in favour of industrial action, but this would be fiercely contested by unions, which say the UK already has some of the most restrictive rules governing strike ballots among developed economies.

The high threshold means that Unison, which balloted more than 400,000 members across the NHS, won a mandate to take action in just 7 NHS trusts out of more than 200, while the RCN strikes will affect just over half of hospitals across England.

This means unions have to take a more tactical approach to maximise disruption, with a general strike practically out of the question.

Additional data analysis and charts by Oliver Hawkins and Chris Campbell