Shadow Chancellor Rachel Reeves has promised that if Labour wins the general election there will be “no further tax increases” beyond those she has proposed.
In his first major speech of the election campaign, Mr Reeves said all of Labour's policies would be “fully funded and costed, with no strings attached, and no buts”.
This includes recruiting thousands of extra teachers and introducing 40,000 NHS appointments per week.
But Chancellor Rishi Sunak insisted Labour's plans would mean “every working household” would have to pay £2,000 each.
He said the analysis was based on Treasury officials “crunching the numbers.”
Speaking at engineering giant Rolls-Royce in Derby, Reeves said Labour was “increasingly being seen as the natural partner of business, the party of growth and the party of corporatism”.
She said: “A few years ago we might never have expected to hear such comments from the Labour Party – just think how far we have come in just four years.”
On Tuesday, 121 former and current business leaders signed a letter supporting Labor's economic plans ahead of the general election, saying “it's time for change”.
In a letter published in The Times on TuesdayThe 121 founders, CEOs and former leaders of companies across financial services, retail and manufacturing said Labour was changing and “want to work with business” for long-term growth.
“For too long, our economy has been plagued by instability, stagnation and a lack of long-term vision,” they said.
Labour is borrowing from the Conservatives to get business leaders to endorse its economic plans.
Ahead of the 2015 election, 100 business leaders endorsed the Conservative Party.
One of them, Malcolm Walker, founder of supermarket chain Iceland, plans to support Labour from now on.
Other former Conservative business letter signatories told BBC News they planned to remain silent amid disappointment over Leader Liz Truss's mini-Budget, the Brexit deal and hopes of a change in government.
Among those signing the Times letter are TV chef and restaurateur Tom Kerridge, several small business chief executives, former Heathrow Airport chief executive John Holland-Kaye, JD Sports chairman Andrew Higginson and Wikipedia founder Jimmy Wales.
Other members include David Cleavy, former chairman of Raspberry Pi, the technology company that has just announced plans to list on the London Stock Exchange, and Karen Blackett, UK chief executive of advertising giant WPP.
But Work and Pensions Secretary Mel Stride pointed out on BBC's Today programme on Tuesday that not a single one of the signatories was a chief executive of Britain's biggest FTSE 100 company.
In the wake of Britain's departure from the European Union, many well-known companies have learned that taking sides during a crucial election period could anger customers.
It is also unclear to what extent this group of Labour supporters is representative of business in general, and its industry in particular.
Reeves believes that publicly declaring his support for the Labour Party will demonstrate to the public the party's credibility.
Some Conservative business figures say there is no effort to produce a Conservative pro-business letter like in 2015.
Advisers to Chancellor of the Exchequer Jeremy Hunt are known to be warning businesses about moving towards French-style workers' rights if Labour wins.
Some major retailers have expressed concern about the Conservative government's plans, which include repealing anti-strike laws.
But Wikipedia's Wales told the BBC's Today programme: “We're certainly not talking about a far-left government, we're talking about a sensible, solid centre-right government.”
He added that he had “tremendous confidence” in the Labour Party.
Chancellor of the Exchequer Laura Trott said: “Rishi Sunak and the Conservative Party have a clear plan that businesses can trust.”
“We took bold action to implement the biggest corporate tax cut in modern history.”
“After years of chaos and mismanagement under the Conservative government, businesses want stability and certainty,” the Liberal Democrats told BBC News.
“The Liberal Democrats will launch an industrial strategy to boost investment, reform our dysfunctional business rates system and support our high streets,” a spokesman said.
The Green Party in England and Wales said it would “invest in the technologies we need to ensure a sustainable and safe future”.
“We need to ensure our economy works in a way that protects the climate and allows nature to thrive,” said co-chair Adrian Ramsay.
Drew Hendry of the Scottish National Party accused Labour of “ignoring the most fundamental causes of the UK's economic decline – the UK is broken and Brexit broke it”, adding: “Only the SNP can fight for a future back at the heart of the European Union.”
Welsh Party Treasury spokesman Ben Lake said the party was “committed to investing in the Welsh economy, driving growth and reducing inequality”.