Plum, Chip and CMC up their cash Isa rates: Which is the new best buy?

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Plum* is the latest cash Isa provider to launch a boosted rate for savers, with new customers able to earn 5.92 per cent on their money for three months.

The firm is the latest entrant in a week-long battle to win customers by offering boosted rates, with the Plum Isa leading picks from the likes of CMC Invest*, Trading 212* and Chip*.

Boosted rates give temporary extra interest for a set period, before the rate falls back to a lower level.

The end of the tax year is fast approaching and last-minute savers now have several options for maximising their Isa allowance before the new tax year starts on 6 April.

Plum's* boosted rate looks the most attractive for savers at 5.92 per cent, but plummets after three months to 3.54 per cent.

Its average rate over 12 months works out as 4.14 per cent.

It's also not flexible, meaning you can't withdraw money and replace it without affecting your allowance, and you won't get the bonus rate if you make more than three withdrawals or your balance drops below £100.

CMC Invest* was already a top Isa pick because of its high regular rate of 4.85 per cent. The next best regular rate is offered by Tembo at 4.8 per cent.

Earlier this week we crunched the numbers to reveal these rates could still beat the quartet of providers offering boosted rates, when working out average rates over 12 months.

Using the same method for calculating the average – three months at the higher rate and nine months at the standard rate – those stashing their cash in a CMC Invest Isa could enjoy 5.06 per cent interest over the whole year.

Chip* has a more attractive headline rate of 5.9 per cent but the firm's regular 4.32 per cent rate puts its average below CMC Invest, at 4.72 per cent.

Each of these Isas are flexible, which means you can withdraw money and pay it back in the same tax year without affecting your allowance.

They all let you make unlimited withdrawals without penalty, unlike some other providers that lower your interest rate if you take money out more than three times. You can open them with just £1.

It's worth keeping in mind that all of these rates are variable, so there's no guarantee you'll receive the current advertised rates over a full year.

At the beginning of the week, Isa providers kept inching rates up in an effort to offer savers the most attractive headline number.

In response to CMC Invest's leading headline rate, Chip then pushed its own boosted rate up to 5.9 per cent.

With just over 24 hours to go until the end of the tax year, CMC Invest could be the best option for cash savers who are hunting for top rates over the long-term, and are still undecided about where to put their money.

That's because of the best-buy underlying 4.85 per cent rate savers should receive after the bonus period.

You can read more last minute tips on maximising your Isa or pension before the end of the tax year.

We also have a table of the best cash Isa rates to discover more deals and our regularly updated round-up tells you more about five of our favourite cash Isas.