Three million NatWest and RBS Isa account holders will have their rates cut or boosted, as the banks are moving anyone with an old Isa to a newer deal.

Just over a million savers are being told they will get up to 1% less on their savings, while another 1.9 million people will get up to 1% more.

But don't assume that just because the two banks are boosting some rates, that you're getting the best deal on the market. Check if you can earn more by transferring your cash elsewhere.

The Post Office's Premier Cash ISA Issue 5*, for example, pays 1.8% AER, including a 0.9% bonus for 18 months. See our Cash Isa Transfers guide for the best buys that accept transferred money.

I have a NatWest/RBS cash Isa. What's happening with my rate?

Customers who opened a NatWest Cash Isa or RBS Instant Access Isa after July 2013 are unaffected by the change – their rates will remain at 1% AER on deposits of up to £25,000 and 1.5% AER over £25,000.

Customers who opened a NatWest Cash Isa or RBS Instant Access Isa before July 2013 will see their rates either rise or fall, depending on how much cash is in their Isa, to 1% AER on deposits of up to £25,000 and 1.5% AER over £25,000 on 31 January.

These customers currently earn between 0.5% AER and 2% AER with NatWest and between 0.55% AER and 2% AER with RBS.

If you've got one of the old Isas listed below, you'll be moved to either the NatWest Cash Isa or RBS Instant Access Isa on 31 January 2014. Here, your rate will change to 1% AER on deposits of up to £25,000 and 1.5% AER over £25,000.

Customers who've currently got an e-Isa or Cash Isa Plus with either bank, pay into it regularly and have a current account with the relevant bank will also get an extra 0.3% AER for the first six months after the move.

  • NatWest Cash Isa Tessa Funds – currently pays 0.5% AER
  • NatWest Cash Isa Plus – currently pays 2% AER
  • NatWest e-Isa – currently pays between 1.75% AER and 2.25% AER depending on how much money is in the account
  • RBS Cash Isa Plus – currently pays 2% AER
  • RBS 60-Day Isa – currently pays 0.5% AER
  • RBS Cash Isa Tessa Funds – currently pays 0.5% AER
  • RBS e-Isa – currently pays between 1.75% AER and 2.25% AER depending on how much money is in the account

Both banks, which are part of the RBS group, will write to you over the coming days to tell you if you're affected.

If you don't receive a letter and think you should be getting one, get in touch with your bank's customer service department.

Why are RBS and NatWest doing this?

The banks say the move will put an end to "new customer only" offers and "teaser rates", which often lure customers in with high bonus interest rates that drop after a certain time period.

RBS/NatWest head of savings David Crawford says: "It's important that our Isa savings range is simple and transparent for our customers. The majority will see some benefit to their rate as a result of the changes we're making, some by as much as 1%."

Does the move affect any other T&Cs?

NatWest and RBS say there are no negative changes to terms and conditions as customers are being moved to easy-access Isas.

Currently, some customers with older Isas can only access their account online, or may have limits on making withdrawals or transfers, but the move will lift some of these restrictions.

Does the change affect my Isa allowance?

No. The move only affects the interest rate.

What is a cash Isa?

A cash Isa is a savings account where the interest earned isn't taxed.

At the start of each new tax year, savers are given a maximum allowance they can invest in a cash Isa. The amount changes each year in line with inflation. For the current 2013/14 tax year, the most a cash Isa holder can deposit is £5,760.

Many Isas allow full and instant access to your cash, allowing you to withdraw your money if you need to. However, the maximum amount you can put into an Isa in a tax year remains the same regardless of whether you withdraw any money or not. For more, see our Cash Isa guide.

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