Savers poured a record £14bn into cash Isas in April amid fears of a raid by Rachel Reeves on the tax-free accounts.

Monthly deposits were the highest since the system was introduced in 1999, according to Bank of England data.

While Isa deposits tend to rise towards the end of the tax year on April 5, Ruth Gregory, at Capital Economics, said the record total “was probably due to speculation around the Chancellor considering slashing the cash Isa tax-free allowance”.

Ms Reeves has been consulting on changes to the Isa system as she seeks ways to push more money into stocks and shares to boost growth.

Savers can currently stash up to £20,000 into an Isa every year, with the option to split the money between cash or stocks and shares.

The Chancellor confirmed last month that she will not change the overall annual limit on contributions, although she left the door open to curbing the amount that can be held in cash.

Ms Reeves said: “I’m not going to reduce the limit of what people can put into an Isa, but I do want people to get better returns on their savings, whether that’s in a pension or in their day-to-day savings.”

Emma Reynolds, the City minister, previously told a Lords committee that cash Isas were draining investment from the London Stock Exchange.

“Why have we got hundreds of billions of pounds in cash Isas? We have failed to drive an investment culture,” she said.

Lowering the amount that can be put into cash Isas would mean millions would be able to save less each year tax-free and would face a choice between putting money into savings accounts subject to tax or investing in riskier stocks.

Banks and building societies have been urging Ms Reeves to leave the system as it is.

David Postings, the chief executive of UK Finance, which represents both banks and building societies, told The Telegraph last month: “They are an easy-to-understand product that help individuals start saving and set aside money for the future.

“The money banks and building societies hold in cash Isas is also lent out, supporting borrowers and the wider economy.”

Robin Fieth, the chief executive of the Building Societies Association (BSA), said: “Simply changing Isa limits is unlikely to encourage people to invest, but it will hurt people who are responsibly saving for short-term goals, when investing is not appropriate.

“If the Government decides to make any changes to Isa limits it should make them to both stocks and shares Isas as well as Cash Isas, otherwise the administration of the system will become unnecessarily complicated.”

Roughly 22.3m British adults hold more than £725bn in Isas, according to government data.