NatWest has launched a one-year fixed-rate cash Isa paying 4.2 per cent. 

It’s an attractive rate in today’s market, only outpaced by the 4.27 per cent on offer from app-based Tembo. It’s also an unusual move – we rarely see one of the big banks offering a top rate.

But many NatWest loyal savers could be left out in the cold.

You can’t grab this rate if you have already opened a fixed-rate cash Isa with the bank this tax year, which started on April 6.

NatWest, along with many other providers, only lets you run one cash Isa with them in the current tax year.

You do have the option of closing your current fixed-rate Isa with NatWest but you’ll be stung with an early closure penalty worth 90 days’ interest.

Once you deduct this charge from the 4.2 per cent rate, it is not worth the move. NatWest is far from alone in letting you only open one cash Isa with it during the current tax year. 

This is despite the rules that came in last year that said you could open and run as many cash Isas as you like with your current £20,000-a-year allowance.

Previously you were only allowed to open one cash Isa with each year’s allowance. It was a welcome move at the time but it’s up to each individual provider to decide whether or not they let you run more than one with them. 

Check the best cash Isa rates in our savings tables 

Most still do not have the computer system in place that lets you do this.

And they are unlikely to invest in the way their cash Isas are run if the rumoured cut in the cash Isa allowance comes in the Budget next month.

The result is that if you have already snapped up a good fixed-rate account earlier this year with some of your allowance, you can’t go for another one from the same provider later in the year.

Other big providers which only let you run one Isa with them with this year’s allowance include Barclays, HSBC, Halifax, Lloyds, Santander and Co-op Bank along with building societies Coventry, Yorkshire, Principality and Leeds.

A handful let you mix and match any number of fixed-rate and easy-access Isas with them with your £20,000 allowance in this tax year. 

They include Charter Savings Bank with its Mix and Match Isa, Zopa (Smart Isa), Paragon (Isa Wallet), Nationwide (Portfolio Cash Isa), Newcastle BS (CustomIsa), Aldermore (Maximisa) and Vida Savings.

Bag yourself 4.32% with Shawbrook

Shawbrook Bank has a new version of its Easy Access Bonus account paying 4.31 per cent.

It’s a near-top rate but you need to make a note to move your money after a year when the rate drops off a cliff. 

It comes with a bonus of 2.31 percentage points for 12 months and you only need £1 to open one. But it is only open to new savings customers.

It is only beaten by Cahoot Sunny Day Saver at 5 per cent where the most you can hold is £3,000 and the rate nosedives to 1 per cent after a year.

App-based provider Chip Instant Access also beats it with a tempting 4.8 per cent but it has a 2.33-point bonus which lasts for just three months.

I don’t include accounts boosted by a bonus in my star buy tables, but they are a good option for those willing to move money around often.