A growing number of providers are using short-term bonus rates to boost the headline returns on cash ISAs, with experts warning this could leave savers shortchanged in the future.

Many of these accounts offer the best returns but become far less competitive once the bonus period ends, new analysis has revealed.

Research conducted for Investec Save by Moneycomms found that bonus-dependent accounts now make up half of the top 10 highest-paying instant-access ISAs.

On average, these bonuses add 1.68 percentage points to the advertised rate over the first 12 months, though they vary widely – from as little as 0.49 per cent to as much as 3.16 per cent.

Once the bonus disappears, however, the underlying rate often plunges. Just two bonus-rate accounts remain in the top 50 when judged on

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