Keir Starmer is under fresh pressure over the former Tory MP Natalie Elphicke’s defection to Labour after the president of the Trades Union Congress said her vocal support for anti-strike laws should be “incompatible” with the party whip. Matt Wrack, who is also the general secretary of the Labour-affiliated Fire Brigades Union, has described the MP for Dover and Deal’s views as “disgraceful” after she used a parliamentary intervention in March to blame firefighters for the deaths of three people who perished during a national strike. Wrack’s comments have been set out in a letter sent to Starmer this weekend, which has been seen by the Guardian. Senior Labour figures have been forced to defend Elphicke amid claims she lobbied the justice secretary to interfere in her then husband’s rape case – claims her spokesperson has described as “nonsense”. Wrack, who became president of the TUC in September, wrote in the letter that Labour’s decision to admit Elphicke was “alarming” because of the party’s promise to repeal the Strikes (Minimum Service Levels) Act, which effectively bans strike action across parts of the public sector. “Labour’s pledge to repeal this authoritarian legislation within 100 days of taking office, alongside the 2016 Trade Union Act, is a crucial commitment. It is therefore alarming that Natalie Elphicke has been admitted to the parliamentary Labour party. “Elphicke was a cheerleader for the minimum service levels act and has specifically targeted firefighters in her contributions in parliament. “On Tuesday 12 March this year, she spoke in support of the new anti-union laws by blaming striking firefighters for the deaths of three people during a past national firefighters’ strike. This is a disgraceful attack on firefighters, who protect the public and save lives every day, sometimes at great personal cost,” he wrote. “The Labour party is the political wing of the labour movement … Attacking trade union members in this way to justify support for draconian anti-worker laws ought to be incompatible with membership of the parliamentary Labour party. “Natalie Elphicke should never have been given the Labour whip, but these remarks further undermine the decision to accept her into the party. There appears to have been little, if any, due diligence.” Asked on Friday about unease from senior Labour figures about Elphicke’s defection, Starmer urged his party to be “less tribal”. “I am very pleased to welcome Natalie to the Labour party,” he said. “Natalie’s conclusion, having thought about this profoundly, is that Rishi Sunak has effectively lost control of the borders, the Tory party is characterised by incompetence and the Labour party has changed. I think that is a very powerful thing to have said.” Wrack’s intervention comes as Starmer seeks to capitalise on the political momentum generated by both the recent local elections and Elphicke’s defection. The Labour leader will meet the party’s new slate of English mayors on Monday for the first time since this month’s elections, saying that driving regional economic growth will be top of Labour’s devolution agenda. While Starmer wants voters to focus on the economy going into this year’s general election, the prime minister, Rishi Sunak, will give a speech on Monday in which he will attempt to frame it as a choice between a forward-looking Conservative party and a backward-looking Labour. Sunak is facing a gradual bleeding of his parliamentary authority as Tory MPs line up to announce they are standing down at the next election while Labour works to secure more defections. Elphicke, a rightwing Tory MP, shocked Westminster on Wednesday when she crossed the floor and joined Labour MPs at the beginning of prime minister’s questions. The move initially delighted Starmer’s closest allies, showing that even anti-immigration MPs appear to have lost faith in Sunak’s Rwanda deportation plan and his ability to lead the Tories. But Starmer’s decision to accept her into the party has caused upset on his own benches, given her long history of attacking Labour on immigration issues. Jess Phillips, the Labour MP for Birmingham Yardley, told LBC she would “probably have said no” to Elphicke joining the party and there should be an independent inquiry into the latest claims. Zarah Sultana, the Labour MP for Coventry South, who is from the left of the party, told the BBC: “[Elphicke] was a member of the [Eurosceptic] European Research Group; she voted for Liz Truss in the leadership; she’s at odds when it comes to fire and rehire; she has attacked trade unions and their activities; [she’s] not great on the environment either. So unless she’s had the biggest Damascene conversion ever, I just don’t buy it.” Wrack’s letter, written on FBU-headed paper, said the union would be raising its concerns about Elphicke’s views and her admittance to the parliamentary party through formal channels. During parliamentary scrutiny of strike regulations in March, Elphicke expressed her support for plans to allow fire and rescue authorities to issue work notices forcing firefighters to work during disputes. She told the fire services minister, Chris Philp: “It may be helpful to the minister to note that actually three elderly people were reported to have died in the first national firefighters strike – the one that the minister is referring to – and indeed, more recently, the failure to respond to a call-out in the middle of a strike led to a serious incident that very nearly led to loss of life in Essex. “That might be helpful to the minister, to expand on why it is so important that these measures are put in place to save lives.” The FBU has condemned the claims as unsubstantiated. Union bosses and Starmer will discuss Labour’s pledges on workers’ rights in a meeting on Tuesday. The Guardian disclosed on Wednesday that Unite and the FBU were among unions concerned that Labour was watering down proposals on “fire and rehire”, zero-hours contracts and plans for legislation. Key to the criticism from trade unions were changes to the wording of plans to end fire and rehire – removing a direct promise to end the dismissal of workers for rejecting a worse contract. A union source said: “Elphicke’s move across the floor looks like the direction of travel under Keir. He may have to be reminded on Tuesday that the unions will not be messed about.”