Nicola Sturgeon has challenged Rishi Sunak over his planned anti-strike laws, telling the Prime Minister he should not be pouring fuel on pay disputes by removing workers’ rights.

The First Minister is understood to have criticised the legislation during an informal dinner with the Prime Minister as he made his first visit to Scotland since entering Number 10 to unveil the country’s first green freeports.

She said the Scottish Government would strongly oppose the Strikes (Minimum Service Levels) Bill which was published on Tuesday.

The UK legislation would enforce minimum service levels during action for certain employees, including ambulance workers, rail staff, and firefighters. 

Business Secretary Grant Shapps told MPs it would protect lives and jobs, but unions were scathing, calling it “undemocratic, unworkable and almost certainly illegal”.

Labour said it would repeal the legislation, which would apply in Scotland, England and Wales, if it won the next General Election.

The Bill was introduced by the Tory government as it struggles with waves of strikes by public sector workers and transport unions.

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Ms Sturgeon and Mr Sunak had what Downing Street called an “informal and private dinner” near Inverness tonight.

The two leaders had already met face-to-face in November at the British Irish Council.

The dinner, arranged at just 24 hours’ notice by No10, was intended to discuss “the shared challenges that people in Scotland and across the rest of the UK face – and how both governments can co-operate meaningfully to ensure that they are jointly delivering for people, their families, and communities”.

However, it coincides with cross-border rows over the constitution and Holyrood gender reforms.

Ms Sturgeon has said she intends to fight the next election as a “de facto referendum” if London denies her the power to hold a vote on the same legal footing as the one in 2014.

The UK Government is also considering whether to stop the Gender Recognition Reform Bill passed by MSPs before Christmas becoming law, on the grounds that it cuts across UK equality law. 

Mr Sunak’s first official trip north of the border as PM is also intended to promote the Union with the announcement of Scotland’s first green freeports, funded by £50 million from the Treasury.

The Cromarty Firth and Firth of Forth will now become large-scale enterprise zones able to attract jobs with a series of local tax breaks.

They beat rival bids from the Clyde and Aberdeen areas and Orkney.

Mr Sunak will say the freeports, which are joint UK and Scottish Government projects, will “create thousands of high-skilled green jobs, drive growth, and potentially bring in billions of private sector investment and provide opportunities for people across Scotland”.

Freeport critics, including Ms Sturgeon’s Green ministers, argue they merely displace jobs from elsewhere and “hand tax breaks and public money to rich corporations”.

Mr Sunak also met search and rescue workers and a community group during his visit.

Ms Sturgeon made clear her intention to confront Mr Sunak over anti-strike laws at FMQs yesterday, when SNP MSP Paul McLennan asked her to condemn the “brazen attack on trade union rights”.

She said it was “a really important issue” and noted the UK already had “the most anti-trade union laws in western Europe”.

She said: “The proposed Bill threatens to undermine and weaken the rights of workers even further. 

“We strongly oppose any bill that undermines legitimate trade union activity and does not respect fair work principles.

“As Governments, we should be working with the public sector and trade unions to reach fair and reasonable settlements that respect the legitimate interests of workers, rather than pouring fuel on fires or taking away workers’ democratic rights. I will make those points very strongly when I see the Prime Minister.”

Read more: Sturgeon 'not planning' to raise Holyrood gender reforms with Sunak

Her official spokesman later said the cost of living crisis and NHS pressures were also likely to be discussed, however Ms Sturgeon would not be raising the Gender Recognition Reform Bill (GRRB) as she considered the matter settled. 

He said the constitution “may come up”, although that was uncertain, despite the long-running dispute over Indyref2.

The UK Government must decide next week whether to block the GRRB by making an unprecedented order under Section 35 of the 1998 Scotland Act denying it royal assent.

In theory, this would be to pause the Bill so that Holyrood could revise it, but Scottish ministers are far more likely to go to court in a bid to secure the Bill as it stands. 

Asked whether the GRRB would be raised, the spokesman said: “It’s been passed comprehensively by the Scottish Parliament on a cross party basis, so not planning to raise it. It’s up to the PM whether he wants to raise that issue or not. 

“If the PM raises it, obviously the FM will respond and we’ve already made our views on that very clear – the fact that it is within [legislative] competence, that it’s been comprehensively passed by the Scottish Parliament, and any attempt to undermine the democratic will of the Scottish Parliament will be vigorously contested.” 

Asked if the Scottish Government welcomed the dinner invitation, he said: “It’s always welcome to have open lines of communication and discussion. Face to face meetings are always welcome.”

Mr Sunak’s predecessor Liz Truss refused to meet or call Ms Sturgeon while she was briefly in No10. Nor was there any love lost between Ms Sturgeon and Boris Johnson.

The Prime Minister’s spokesman said whether to make a Section 35 order was being looked at, adding: “There is a process to consider it ... that’s still taking place.”