The bank, which is backed by the Treasury, is cutting interest rates on 1.5m accounts which contain almost £25bn worth of savings. Experts have blamed the move on a no-deal Brexit.

NS&I announced that it will cut interest rates by 0.25 percentage points for all customers when fixed-rate terms mature on Guaranteed Growth Bonds, Guaranteed Income Bonds or Fixed Interest Savings Certificates.

Simultaneously, it has said that it is closing to new customers the one and three-year fixed-rate editions of its Guaranteed Growth Bonds and Guaranteed Income Bonds. This move has come just over a year after it cut the amount of money newcomers could deposit into these accounts from £1 million down to £10,000.

The accounts affected are among the most popular in a British savings market ruined by poor interest rates. Collectively, there are 1.46 million accounts held within these three products with a collective total of £23.9 billion of cash.

The bank faces extensive pressure to cut back to remain within the Treasury’s stringent net financing limit. yesterday, the bank said that it was making the changes for numerous reasons, including falling rates for similar products offered by competitors.

Analysts have indicated that the increasing likelihood of Britain leaving the European Union without a deal was also motivating the bank’s decision. this has been fuelled by hints from Mark Carney, governor of the Bank of England, that the base rate might be cut even further.

Andrew Hagger, founder of the personal finance advice site Moneycomms, said: “No-deal Brexit uncertainty isn’t helping this situation. The next Bank of England rate change is likely to be downwards and that’s certainly the direction high street providers will be heading.”

Mr Hagger said he believed that NS&I bosses are worried that as the political climate becomes more tense, its reputation as a safe haven, due to it guarantee of 100% security for up to £1 million of savings instead of the £85,000 limit for regular high street bank accounts, could attract many new customers. This rush to join the bank could break the NS&I limit.

He added: “There are going to be a lot of people sitting on a big sum of money in those bonds. It’s almost like gold — people rush towards safe havens in times of uncertainty and this could be the banking equivalent.”

The change will have a sharp impact on the returns of those who have accounts with the bank. It means, for example, that the rate on a one-year Guaranteed Growth Bond will decrease from 1.5 per cent to 1.25 per cent when the account matures.

Contrastingly, on a one-year Guaranteed Income Bond, the rate would go from 1.45 per cent to 1.2 per cent, and a two-year Fixed Interest Savings Certificate will dip from 1.55 per cent to 1.3 per cent. The new rates affect customers whose deals mature after October 5.

Personal finance analysts suggest that savers should leave the bank to receive better rates when their accounts mature.