Keir Starmer has used a series of new workers rights that come into force on Monday to attack the Green party, saying a vote for Labour’s rivals puts such progress on sick pay, parental leave and zero-hours contracts at risk.

The prime minister also took a swipe at business figures and opponents of what he described as the biggest strengthening of workers’ rights in a generation, dismissing “vested interests” who had warned against them.

However, in a sign of how he views the threat from the populism of Zack Polanski’s Greens and Nigel Farage’s Reform UK in the run-up to local elections in May, Starmer said that having “a serious, credible economic strategy” set Labour apart from others.

“No other party offers both the economic credibility and the political will to do this,” he wrote in an article for the Guardian.

“A vote for any other party puts that progress at risk – whether through choices that would take us backwards, or approaches that simply don’t stand up to the realities of governing.”

Measures that come into force on Monday include the lifting of the two-child benefit cap, a key demand of child poverty campaigners and Labour MPs. Starmer described the move as one of the proudest moments of his government.

Other measures coming into force on Monday include a 4.8% increase in the state pension to £241.30 a week and a 2.3% rise in universal credit standard allowance.

Under the Employment Rights Act 2025, statutory sick pay becomes a right from day one of becoming ill. Workers will also become entitled to paternity and unpaid parental leave from the first day of employment.

Labour is eager to brandish the measures as significant achievements as it braces itself for potentially heavy losses in English council and mayoral elections on 7 May amid challenges from Reform on the right and the Greens on the left. There are also elections in Scotland and Wales to their national parliaments.

While Labour has been concerned about Reform since the general election, it has also increasingly been turning its sights on the Green party since the latter won the previously safe Labour stronghold of Gorton and Denton.

The latest YouGov polling on Westminster voting intention had the Greens in joint second place, behind Reform. Polling released on Sunday by the former Tory treasurer Michael Ashcroft indicated a three-way split between the Greens, Conservatives and Reform. Each were on 21%, with Labour on 17%.

A spokesperson for the Green party responded to Starmer’s comments by saying it was now the party of the working class. “This is desperate from our caretaker prime minister, who woke up to a poll this morning showing Labour in fourth place and the Greens in first.

“The truth is that Labour had to be dragged into giving new workers rights which were watered down after corporate lobbying worked on them. The disgraceful two-child cap was only ended after Starmer was finally dragged into it by pressure from Green MPs and anti-poverty campaigners.”

Starmer’s comments appear to affirm a recent pivot to the left, amid pressure from potential leadership contenders including Angela Rayner and Andy Burnham.

“At every stage, we faced those same voices of opposition,” the prime minister wrote of the measures, which had been met with resistance from some business leaders. That opposition had focused on so-called “day one rights”, which give more power to workers to claim unfair dismissal and request flexible working.

Starmer said: “They warned of costs, of disruptions, and said the time was not right. But once again, we made a different choice. We chose working people.

“Nothing Labour has achieved came easily. Every success was hard fought and hard won against the pull of vested interests. And each time, those warnings were proven wrong.”

The prime minister presented the introduction of a series of measures in the tradition of the Blair government’s introduction of the minimum wage 27 years ago this week.

At the same time, Starmer’s leadership has also continued to face trenchant criticism on the left from Unite, traditionally one of Labour’s largest trade union backers. Its general secretary, Sharon Graham, has described the Employment Rights Act as “a shell of its former self”. Last month, the union significantly cut membership fees to Labour, mainly over the Birmingham bin strike.

The scrapping of the two-child benefit cap, which was was introduced in the 2015 budget by the then Conservative chancellor, George Osborne, was criticised by the Tories, who said it would cost billions and “reward worklessness”.

The party published what it said was analysis showing that at least £1bn extra every year would go to 186,000 workless households, with a family of two unemployed adults and three children standing to receive a £6,400 income increase.

It added that the gains were heavily concentrated in a handful of cities, with Leeds, Manchester, Birmingham, Bradford and Glasgow to receive more than £200m more annually.

“While working people struggle with rising fuel costs and food prices, Keir Starmer is giving another handout to those on benefits,” said the Tory leader, Kemi Badenoch.

Labour accused the Tories of issuing “bogus numbers” by using a family with two disabled adults as a case study and pretending they were unemployed. Both adults were listed as receiving the limited capability for work-related activity element of universal credit, meaning they had a disability or health condition limiting their ability to work.