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.