If you've opened a fixed-rate ISA since 6 April, you should hurry to top up your savings if you can afford to do so, as many providers have opened a short window for new money to be deposited following today's ISA revamp.

From today, ISAs have become NISAs (New ISAs), which means you can deposit £15,000 tax-free between cash and stocks & shares. See the ISAs become bigger tax-free NISAs today MSE News story for a full round-up of what the changes mean.

But while the new higher limit is a boon for some savers, if you opened a fixed-rate cash ISA between 6 April and 30 June this year – before it turned into a NISA today – check if you can take advantage of the bigger tax-free savings allowance.

Fixed-rate NISAs mean your cash is locked away for a certain period of time – and often, you're only able to make one deposit.

But most providers are allowing people to take advantage of the new rules to boost their fixed rate NISAs from the old £5,940 maximum to the new £15,000 allowance.

These are the fixed rate ISAs we've featured on MoneySavingExpert.com since 6 April that allow top-ups:

Fixed rate NISA top-up windows
Provider Top-up window
Coventry BS Until 31 July 2014, or until the NISA is withdrawn from sale.
Halifax

Account opened 11 March-30 June: 180 days from account opening.
Account opened on or after 1 July: 60 days from account opening.

Julian Hodge Until 31 July 2014.
Kent Reliance Until the NISA is withdrawn from sale. (i)
Leeds BS Until the NISA is withdrawn from sale.
Lloyds Until 5 April 2015.
Nationwide Until 31 July 2014. (i)
Newcastle BS Until the NISA is withdrawn from sale. (i)
Principality Until the NISA is withdrawn from sale.
Santander Until 31 August 2014.
Skipton Until the NISA is withdrawn from sale.
Tesco Until 31 July 2014.
TSB Until 5 April 2015.
Virgin Up to 30 days from when the NISA is withdrawn from sale.
(i) As long as you don't use up the full £15k allowance, after the window closes you can open a new cash NISA with the same provider and pay new money into this instead. See the Split your NISA MSE News story.
Martin Lewis
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What happens once these top-up windows close?

Once the top-up window closes, you can no longer deposit money into that cash NISA or any other cash NISA this tax year.

Some providers, including Nationwide, Kent Reliance and Newcastle Building Society, offer a loophole that allows you split your cash between different products with the same provider, so long as you don't deposit more than the £15,000 annual allowance. See the Split your NISA MSE News story.

You can also pay any remaining allowance into a stocks & shares NISA. See our Stocks & Shares guide for more info.

What if my fixed-rate NISA doesn't have a top-up window?

If your fixed rate NISA doesn't have a window to deposit new cash, you can't pay any more money into that cash NISA or any other cash NISA this tax year.

But you can still put your remaining allowance into a stocks & shares NISA.

Can I open a new fixed-rate NISA?

If you haven't put any money into a cash NISA (or ISA) since 6 April this year, you can open a brand new cash NISA. See our Top Cash NISAs guide for the best buys.

I have an easy-access NISA, can I top that up?

Yes, you can top that up throughout the tax year, so long as you don't go over the £15,000 allowance.

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