Isa providers are making it difficult to gain interest

However, many leading providers are doing their best to reduce savers' returns even further.

Unlike normal savings accounts, where interest earned by basic-rate payers is taxed at 20%, gains on cash Isas (individual savings accounts) are tax-free.

Because of this, there are strict savings limits. Everyone aged 18 or over can open and pay into just one cash Isa each tax year.

The allowance for this year, which ends on April 5, is £5640. For the new year, which starts the following day, it is £5760.

The minimum age for opening a cash Isa is 16. As well as benefiting from the adult allowance, 16 and 17-year-olds can pay £3600 into a separate junior Isa (£3720 from April 6), giving them a total tax-free allowance of up to £9240 (£9480 from April 6).

Financial experts have long advised anyone with spare cash to make maximum use of Isas. Anyone who hasn't invested their 2012-13 allowance has less than three weeks to act.

But this year many providers are doing their best to cut the benefits to hard-pressed savers.

Short-term interest bonuses have, for some time, been a major feature of the savings market, and Isas are no exception.

Savers are enticed with an attractive rate, but this falls dramatically when the introductory period ends.

Because many don't notice or can't be bothered to take their money elsewhere, the provider gets to hang onto the cash while paying far less interest.

MoneySupermarket.com has calculated that this "bonus apathy" could cost savers as much as £500 million this year.

Kevin Mountford, the comparison site's head of banking, said: "If you have funds in an existing Isa and have held this for more than 12 months, the chances are you will be receiving a rate far lower than you can receive on the current leading products in the market.

"It is, therefore, vital that savers check the current rates paid on their Isa and be prepared to switch. The difference between the average and top-paying rates can be significant."

That is all very well in theory, but, in the past, most providers accepted previous years' balances held with other financial institutions, which do not count towards the current year's allowance. This enabled savers whose existing rate was no longer competitive to move their entire cash pile.

Many leading firms have now banned transfers in, dramatically reducing the interest they will have to pay out, including 'best buy' providers Coventry Building Society, Tesco Bank, National Savings & Investments and GE Capital Direct.

Sylvia Waycot, finance expert at comparison service Moneyfacts.co.uk, warned: "The silent withdrawal of transferable Isas may mean that savers need to rethink how they view Isas.

"Traditionally, savers changed providers yearly, taking all their cash with them. The relationship had been based entirely on rate, but, if providers are closing the door to transfers, savers may find they are stuck in accounts that have lost their appeal.

"With two-thirds of the top-paying Isas and one-fifth of variable Isas not allowing transfers in, savers with the largest pots are already being turned away, leaving them either with the same provider as last year, in an account that could be paying peanuts because the bonus has expired, or with a new account that is not a best buy."

It is also important to read the withdrawal terms, as there may be an interest penalty for failing to give enough notice.

Traditionally, savers who agreed to tie up their cash for a set term received a better return, but this is no longer the case. One-year fixed Isas currently pay up to 2.25%. The top two-year rate is 2.8% and those willing to leave their money for three to five years can make around 3%.

With similar returns on variable rate products, these deals are no longer anything special.

Sylvia Waycot advised: "Savers need to find an Isa that not only pays a great rate but is also from a provider that they are comfortable doing business with for longer than 12 months, lest they become Isa prisoners for life."