The Institute for Public Policy Research, a think tank, has been campaigning for a rise in capital gains tax and it is understood that Labour had expressed an interest in the idea.

It has previously suggested several policies that have later been adopted by the party, including a windfall tax on the excess profits of energy firms and increasing public funding to achieve net zero.

Labour’s “grey book”, which it published alongside its last manifesto in 2019 to explain how it would fund its spending commitments, pledged to tax capital gains at the same level as income tax.

The party is yet to announce which measures it will keep ahead of the next election. However, Sir Keir has already said he intends to pursue his predecessor Jeremy Corbyn’s policy of axing the financial benefits that private schools derive from their charitable status.

Several Labour backbenchers are pushing for an increase in capital gains tax - including Richard Burgon MP, who has presented a petition to Parliament on the issue.

“This is a campaign that we are pushing as backbenchers,” he said. “There has been a lot of interest and a lot of support from Labour MPs from different parts of the country.”

The Labour Policy Forum, which has been conducting a consultation on policies to feed into its next manifesto, has received several submissions from Labour branches calling for capital gains tax to be reviewed.

The party has said it will reveal its policies on taxes closer to the next election.

“In relation to tax, obviously we will set out our position as we go into an election,” said Sir Keir on Thursday.

“In the end, that goes back to the point I made about the choices that are made. When this Government has had to raise taxes it has always gone for the same people, which is working people, and I think that is now writ large in relation to what we have seen in the last 24 hours.”

Meanwhile, Ms Cooper also failed to rule out equalising capital gains tax with income tax. 

She told the BBC: “The point that Angela is making is a broader one, about the way in which the Conservatives have always ended up cutting the taxes for those who were on the highest income at a time when everybody else is ending up being squeezed and everybody else is ending up paying more and I just think that’s not fair.”

'Catastrophic' for business

On Thursday night, senior Tories warned that a raid on capital gains tax would stifle enterprise and be “catastrophic” for business.

Sir John Redwood, a former Tory cabinet minister who ran Margaret Thatcher's No 10 policy unit, said: “Britain needs more enterprise, not less. It needs more people prepared to take risks, create jobs and make money and capital gains tax is a big turn-off.

“Labour has in the past been very anti-enterprise. The last thing we want is to make it more difficult to innovate, create jobs and create new business.”

Lord Hammond, the former Tory chancellor, told The Telegraph: “Any uncertainty around capital gains tax regime, particularly for early-stage businesses, would be catastrophic.

“If Labour go into the next election with anything like the policy they had at the last election, which is the equalisation of rates, it would be completely catastrophic for the venture capital sector trying to innovate and grow.

“There’s a real concern that possibly they may not understand how important the capital gains tax regime is to make the UK a credible location for early-stage, high-growth potential businesses.”

A Labour spokesman said: “We have no plans to raise capital gains tax. Working people are facing the biggest fall in living standards on record yet the Government refuses to scrap non-dom status, put in place a proper windfall tax, or end tax breaks for private schools.”