Concern about the stability of ING, the Dutch bank, which may see it downgraded by Moody’s, the credit rating agency, has sounded alarm bells for savers.

More than a million people in Britain have savings with the bank’s online arm, ING Direct. Thousands more had their cash transferred to ING last year when it bought savings accounts from Kaupthing Edge and Heritable, the failed Icelandic banks.

ING is offering existing customers 3 per cent interest on its Growth Saver if they deposit between £50,000 and £1m. Its best-buy easy access account at 3.2 per cent and fixed-rate saver at 3.25 per cent also accept up to £1m.

However, the Dutch deposit guarantee scheme, which would cover British account holders if ING got into difficulties, guarantees only up to €100,000 (£91,745).

West Bromwich Building Society was criticised in May for launching a best-buy savings account for deposits of more than £50,000 after its credit rating had been downgraded. The UK’s Financial Services Compensation Scheme only guarantees deposits up to this amount.

Moody’s could take up to 90 days to decide whether to downgrade ING, previously one of the strongest banks in Europe. The rating gives other financial institutions an indication of how safe it is to lend money to ING.

The European Commission is investigating the Dutch Government’s agreement to cover 80 per cent of ING’s €27.7 billion risky Alt-A mortgages in exchange for a fixed fee. Moody’s fears that the inquiry could lead to higher fees for the coverage, which would sap the bank’s depleted reserves.

Finance experts said that ING Direct savers should not panic. Kevin Mountford of moneysupermarket.com, the comparison website, said: “The possibility of a downgrade shouldn’t alarm savers; after all, ratings agencies have downgraded entire governments in the past year.

“As long as customers keep their balances below the compensation limit, they should have nothing to fear.”

ING said: “Our credit rating is in line with other major European and American banks, and there is no reason for customers to be concerned.”