Britain faces a cost of living bombshell to cover the cost of Labour's French-style union laws, voters were warned.
Working from home rights, a massive pay hike for teenagers and sick pay for people who only work a few hours on flexible contracts will leave businesses reeling, Tories claimed.
Labor insisted the analysis of the cost of its radical workers' rights reforms was “nonsense”.
It comes as “slippery Starmer” repeatedly refused to rule out tax rises to cover the cost of his plans for the country.
Treasury Minister Bin Afolami said Sir Keir's sums do not add up and warned voters they will pay the price.
Writing in the Daily Express, he said: “Every time he's on TV, Sir Keir transforms into Slippery Starmer when he's asked to rule out tax rises. Because he can't.
“He knows just as well as we do that life's more expensive under Labour.”
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Labour's deputy leader Angela Rayner has drawn up a “new deal” for workers that she wants to introduce in the first 100 days of a Labor government.
It includes plans to create a single status of “worker” to end self-employment in the so-called gig economy – firms such as Uber and Deliveroo.
Ms Rayner said in February she wants to give workers rights “from day one” in the job as well as guaranteed hours.
Conservative analysis of the measures included in its green paper put the cost at £41 billion a year.
But the claims sparked a furious slanging match with Labour, which claimed the figures were wrong while the Conservatives insisted their research was robust and showed the country faces paying a high price for the changes.
A Labor spokesperson said: “This is absolute nonsense. The Conservatives have once again costed a policy that isn't Labour's.”
“Labour's 70 new French-style union laws will cripple British businesses and, as ever, leave the public to pick up the bill.”
Tories warned that major employers like Tesco face being hundreds of millions a year worse off under the reforms, potentially driving up food prices for families.
The party warned the increased cost of employment could result in higher prices, with labor costs contributing to 40% of the increase in inflation last year.
Ms Rayner's plans have caused tensions in the Labor party, with shadow chancellor Rachel Reeves suggesting the plans could be tweaked after concerns from business.
M&S chairman Archie Norman warned last month the new deal “reduces flexibility, makes it more costly to hire people and seeks to bring unions back into the workplace”.
“Any incoming government should consider carefully whether a package that reduces flexibility, makes it more costly to hire people, and seeks to bring unions back into the workplace will help attract new investment,” he added.
The analysis puts the cost of raising the minimum wage for the under 21s at tens of millions a year.
Giving all workers, including the self-employed, statutory sick pay is expected to run into b
billions a year.
The party also warned of a productivity drop linked to working from home and a huge bill for ending zero hours contracts and covering the pension costs for putting the workers on fixed agreements.
Other costs include extending maternity and paternity leave and increased time off in working hours for trade unionists.
Meanwhile, Sir Keir repeatedly refused to rule out putting up taxes to pay for his spending plans.
He said: “On the question you raised about tax and spending. We absolutely set as our priority economic growth, which is the missing ingredient for last 14 years.”
And the Labor leader was accused of “smashing up” his promises to scrap tuition fees after shadow education secretary Bridget Phillipson refused to rule out actually increasing them.
Rishi Sunak explains why he called a general election
Rishi Sunak signaled the door is open for former prime minister Boris Johnson to campaign for the Tories as he continued his campaign with a visit to Northern Ireland.
The Prime Minister said: “I am very proud of the record of the Conservative Government over the last 14 years. And of course we haven't got everything right and circumstances have been difficult, but there's an enormous amount to be proud of.
“And when it comes to Boris he was of course the person who got Brexit done, ensured we had the fastest vaccine rollout in Europe and was the first to make sure that we provided support to Ukraine.
“And I think all Conservatives will want to join the campaign because all Conservatives are united in wanting to see a Conservative government at the next election, because it's the Conservatives that have got a clear plan that we're working towards.”
Mr Sunak insisted he was “up for the fight” but admitted there were also “difficult days”.
He said: “I love talking to people, I love having the debate, I love having the Q&A with people, answering their questions, making sure they know what I'm about and I'm really confident that over the next few weeks we 're going to have a really good conversation as a country about the future we want.
Sir Keir will today (SAT) accuse Mr Sunak of making Britain poorer while he gets richer, claiming the typical household is now £5,883 worse off since 2019.
He will claim energy costs, council tax, mortgage, taxes and motoring are behind the bill.
Darren Jones, shadow treasury minister, said: “Rishi Sunak's economic failure has meant soaring costs for regular expenses leaving families almost £6,000 worse off since 2019, while he personally has become richer than the King.”