Investec’s latest Savings Index reveals that of the 924 accounts reviewed, 151 feature a ‘bonus’ rate that expires, on average, in less than nine months.

The index, which analyses savings accounts for balances of £25,000 or more, shows that not only has the number of accounts reliant on bonuses to boost their rates increased, but so too has the average bonus size.

Furthermore, the average lifespan of the bonus has shortened from 284 days in October 2007 to 265 days.

Investec argues that this could lead to many savers leaving cash languishing in underperforming savings accounts once their initial bonus rate has expired. However, Investec points out that savers do not need to rely on bonuses to ensure good rates of return. For instance, the Investec High 5 Account guarantees always to pay the average of the top five best-buy accounts as chosen and published by Moneyfacts.

Linda McBain, head of banking at Investec Private Bank, says, ‘Banks are more frequently boosting their headline savings rates using appealing short-term bonuses. While these can seem attractive, the reality is that savers need to take a long-term view.

‘It is impractical to switch savings accounts continually to take advantage of introductory bonus rates. In reality inertia usually takes control and, having been attracted by the initial headline rate, many savers will leave their cash in the same account regardless of what happens to the rate. It is imperative that savers find an account that not only offers a competitive rate but also guarantees consistency.’