Building society chiefs will this week intensify their protests against the chancellor’s plans to cut cash ISA limits by warning that it will push up borrowing costs for homeowners and businesses.

Sky News has obtained the draft of a letter being circulated by the Building Societies Association (BSA) among its members which will demand that Rachel Reeves abandons a proposed move to slash savers’ annual cash ISA allowance from the existing £20,000 threshold.

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The draft letter, which is expected to be published this week, warns the chancellor that her decision would deter savers, disrupt Labour’s housebuilding ambitions and potentially present an obstacle to economic growth by triggering higher funding costs.

“Cash ISAs are a cornerstone of personal savings for millions across the UK, helping people from all walks of life to build financial resilience and achieve their savings goals,” the draft letter said.

“Beyond their personal benefits, Cash ISAs play a vital role in the broader economy.

“The funds deposited in these accounts support lending, helping to keep mortgages and loans affordable and accessible.

“Cutting Cash ISA limits would make this funding more scarce which would have the knock-on effect of making loans to households and businesses more expensive and harder to come by.

“This would undermine efforts to stimulate economic growth, including the government’s commitment to delivering 1.5 million new homes.

“Cutting the Cash ISA limit would send a discouraging message to savers, who are sensibly trying to plan for the future and undermine a product that has stood the test of time.”

The chancellor is reportedly preparing to announce a review of cash ISA limits as part of her Mansion House speech next week.

While individual building society bosses have come out publicly to express their opposition to the move, the BSA letter is likely to be viewed with concern by Treasury officials.

The Nationwide is by far Britain’s biggest building society, with the likes of the Coventry, Yorkshire and Skipton also ranking among the sector’s largest players.

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In the draft letter, which is likely to be signed by dozens of building society bosses, the BSA said the chancellor’s proposals “would make the whole ISA regime more complex and make it harder for people to transfer money between cash and investments”.

“Restricting Cash ISAs won’t encourage people to invest, as it won’t suddenly change their appetite to take on risk,” it said.

“We know that barriers to investing are primarily behavioural, therefore building confidence and awareness are far more important.”

The BSA called on Ms Reeves to back “a long-term consumer awareness and information campaign to educate people about the benefits of investing, alongside maintaining strong support for saving”.

“We therefore urge you to affirm your support for Cash ISAs by maintaining the current £20,000 limit.

“Preserving this threshold will enable households to continue building financial security while supporting broader economic stability and growth.”

The BSA declined to comment on Monday on the leaked letter, although one source said the final version was subject to revision.

The Treasury has so far refused to comment on its plans.

The government has declined to rule out a “wealth tax” after former Labour leader Neil Kinnock called for one to help the UK’s dwindling finances.

Lord Kinnock, who was leader from 1983 to 1992, told Sky News’ Sunday Morning With Trevor Phillips that imposing a 2% tax on assets valued above £10 million would bring in up to £11 billion a year.

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On Monday, Sir Keir Starmer’s spokesperson would not say if the government will or will not bring in a specific tax for the wealthiest.

Asked multiple times if the government will do so, he said: “The government is committed to the wealthiest in society paying their share in tax.

“The prime minister has repeatedly said those with the broadest shoulders should carry the largest burden.”

He added the government has closed loopholes for non-doms, placed taxes on private jets and said the 1% wealthiest people in the UK pay one third of taxes.

Chancellor Rachel Reeves earlier this year insisted she would not impose a wealth tax in her autumn budget, something she also said in 2023 ahead of Labour winning the election last year.

Asked if her position has changed, Sir Keir’s spokesman referred back to her previous comments and said: “The government position is what I have said it is.”

The previous day, Lord Kinnock told Sky News: “It’s not going to pay the bills, but that kind of levy does two things.

“One is to secure resources, which is very important in revenues.

“But the second thing it does is to say to the country, ‘we are the government of equity’.

“This is a country which is very substantially fed up with the fact that whatever happens in the world, whatever happens in the UK, the same interests come out on top unscathed all the time while everybody else is paying more for getting services.

“Now, I think that a gesture or a substantial gesture in the direction of equity fairness would make a big difference.”

The son of a coal miner, who became a member of the House of Lords in 2005, the Labour peer said asset values have “gone through the roof” in the past 20 years while economies and incomes have stagnated in real terms.

In reference to Chancellor Rachel Reeves refusing to change her fiscal rules, he said the government is giving the appearance it is “bogged down by their own imposed limitations”, which he said is “not actually the accurate picture”.

A wealth tax would help the government get out of that situation and would be backed by the “great majority of the general public”, he added.

His comments came after a bruising week for Prime Minister Sir Keir Starmer, who had to heavily water down a welfare bill meant to save £5.5bn after dozens of Labour MPs threatened to vote against it.

With those savings lost – and a previous U-turn on cutting winter fuel payments also reducing savings – the chancellor’s £9.9bn fiscal headroom has quickly dwindled.

In a hint of what could come, government minister Stephen Morgan told Wilfred Frost on Sky News Breakfast: “I hold dear the Labour values of making sure those that have the broadest shoulders pay, pay more tax.

“I think that’s absolutely right.”

He added that the government has already put a tax on private jets and on the profits of energy companies.