The Principality Building Society increased the rate of its online ISA by 0.3% to 1.3%. The news comes after the Bank of England raised the base rate from 1% to 1.25%.
An account can be opened with just £1 and customers can make as many deposits as they want.
Eleanor Williams, finance expert at Moneyfacts.co.uk, said: “Savers looking for a flexible account and wanting to use their ISA allowance may well be tempted by this deal as it allows unlimited deposits.
“This account improves its position in our top charts after the update, however, savvy investors may wish to plan for a review, as the bonus portion of the rate expires after 12 months and the rate paid will then decrease.
“Overall, the account receives an Excellent Moneyfacts product rating.”
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Earlier this week, Money Saving Expert founder Martin Lewis gave this week’s roundup of the best rates on savings accounts right now.
For the most basic easy-access savings accounts, the highest rate is Virgin Money’s M Plus, at 1.56%, he said.
This applies to balances up to £25,000, with a rate of 0.75% on any amount over this limit.
Savers who switch banks can also earn loyalty points for 12 bottles of wine.
Mr Lewis also recommended the Chase savings account, at 1.5%.