Millions of savers need to act soon – or else see the interest rate on their cash Isa plunge to as low as 0.75 pc.

Failing to switch to a better account could cost savers dearly as a slew of top-paying deals have been launched in recent days offering 4.25 pc or more.

Such is the competition in the savings market that NS&I has hiked the rates on its British Savings Bonds, with its one-year rate rising from 4.07 pc to an eye-catching 4.5 pc.

Nearly two million one-year fixed-rate cash Isas worth £53billion are coming to the end of their term – and many sneaky savings providers will simply dump your cash in their terrible easy-access accounts when your fix matures.

With big providers such as Halifax and Lloyds, the rate on your fixed-rate cash Isa will plummet to as little as 0.75 pc if you don’t move your money – with Santander it’s 0.9 pc.

Meanwhile bonuses on a number of tempting easy-access accounts on sale a year ago are about to expire, which will see rates on those accounts drop as well.

When looking for a new home for your Isa money, don’t sign up with your existing provider for another year without looking at what’s on offer elsewhere.

There are plenty of deals paying more than 4.25 pc. You can earn a better rate now than this time last year even though the base rate has fallen from 4.75 to 3.75 pc – although it could rise again from tomorrow when the Bank of England meets to decide whether it needs to raise it to curb rising inflation.

This time last year the best you could do was 4.25 pc on a one-year fixed rate Isa. Now the top rate is 4.5 pc even from providers such as HSBC, Santander and Leeds Building Society, while Skipton has launched an 18-month rate with a top 4.55 pc which allows transfers in.

While the new NS&I rates look eye catching, they are unlikely to be a good option for savers looking for a good home for their cash as their existing fixed-rate cash Isas mature.

That’s because these accounts – also known as Guaranteed Growth Bonds – are not Isas, so interest earned may be subject to tax if you’ve already exceeded your personal savings allowance.

NS&I only offers two cash Isas – its Direct Isa and Junior Isa – and, with rates unchanged at just 3.5 and 3.55 pc, neither are competitive.

However, for those who have already exceeded their Isa allowance, the Guaranteed Growth Bonds might be worth a look.

The 4.5 pc one-year fix is among the best on offer – the top rate at the moment is 4.66 pc from MBNA Bank.

NS&I’s two-year rate is up from 3.98 to 4.48 pc while its three-year is up from 4.02 to 4.45 pc and five-year 4.05 to 4.4 pc.

Beware of taking on a longer-term Guaranteed Growth Bond though.

NS&I lumps all the interest you earn over the term together and pays it out once at the end. As a result it is all taxed in that final year – rather than spread out over multiple years – so increases the risk that you breach your personal savings allowance.

There’s no such problem with the Guaranteed Income Bonds because they pay interest monthly. The rates now stand at an attractive 4.41 pc for one year or 4.4 pc for two years. Longer term it’s 4.37 pc fixed for three years or 4.32 pc for five.

NS&I has not increased the rate on its easy-access Income Bonds, which pay 3.05 pc. This means a huge gap has opened up between the rates on the easy-access and one-year bonds.

If you have savings in its easy-access Income Bond and can afford to lock it away for a year, opting for a one-year Guaranteed Income Bond could be a sharp move.

If you had a £100,000 savings pot, you would earn £254.15 a month in the easy-access bond versus £367.52 in the one-year Guaranteed Income Bond – an increase of £113.37 per month, or £1,360.44 a year.

On £250,000, you would earn an extra £3,399.96 a year and, on £500,000, £6,799.92 a year.

If you don’t need to touch the money for a year, it’s a no-brainer.

Finally, NS&I has also raised the rate on its Investment Account from 1 pc to 2.05 pc – the first rise for nearly three years.

This is a postal account. Despite the rise, you should still move your money out to a better paying account.

Even the NS&I Direct Saver – which you can run over the phone – pays a better 3.05 pc.