By Paul Lewis
BBC Radio 4's Money Box
Savers have been warned to beware the most tempting cash Individual Savings Account (Isa) offers, as some have hidden traps which could cost them dear.
Rachel Thrussell says many deals have "strings" attached
There are less than three weeks left to use this year's tax-free Isa allowance.
The last day to put money into a mini-cash Isa, up to this year's £3000 limit, is 5 April, and banks and building societies are trying to persuade people that their deal is the best.
But Rachel Thrussell, head of savings at price comparison service Moneyfacts, told BBC Radio 4's Money Box that the best rates of interest come with strings.
"The 5.3%, for example, from Saffron Walden, a mini cash tracker Isa, has got a 0.8% bonus for six months, and then it reverts to base rate which is 4.5% at the moment.
"But you should be able to get around 5% on an Isa with no strings attached," she said.
Ms Thrussell told the programme that short-term bonuses - a higher rate of interest for a few months - are very common.
"Bonuses usually last for around about six months, though there are some around for about a year," she said.
"On average, the bonuses tend to be around 0.5%, or even 0.8%, and 0.9%, which is quite a big drop [when they end]."
But Ms Thrussell said there are some good deals around with no strings.
"Halifax has got an internet and phone account paying 5% from the first pound. That is a no-notice account, and there is no penalty to transfer.
"Bradford & Bingley is paying 5% on an internet account that starts from £1000, but again there are no penalties," she said.
Penalties can take two forms.
Accounts can demand a period of notice 30, 60 or even 180 days before you can take money out.
And some impose a fee if you move the money to another account.
Ms Thrussell said that is the worst trick they play.
"The Saffron Walden and Portman have got the two highest paying rates at the moment: 5.3% and 5.25% on the Portman.
"They are 30 day and 25 day notice respectively, and they have both got a £30 fee," she said.
With the Saffron Walden account, "If you decide you don't want to stay in that product you have to give 30 days notice or lose 30 days' interest, plus pay a £30 penalty fee," she continued.
That would wipe out any advantage you may have got from the extra 0.3% over the penalty free accounts.
BBC Radio 4's Money Box was broadcast on Saturday, 18 March, 2006, at 1204 GMT.
The programme was repeated on Sunday, 19 March, 2006, at 2102 GMT