Millions of Britons are potentially missing out on hundreds of pounds extra income a year because their Cash savings are in accounts paying 1.50% or less. The analysis from Yorkshire Building Society and CACI shows this accounts for as many as 6.9 million instant access ISAs, which have an average balance of around £6,700.

Despite increased rates on offer, the amount held in low-paying ISA accounts has surged by £3billion in the past year. Harry Walker, senior savings manager at Yorkshire Building Society, said: "It's surprising to see such a large amount still sitting in low-paying ISA accounts after a period of significant increases to savings interest in the last two years. This data follows from our analysis earlier in the year, which called for consumers to take time to review their finances."

He suggested that starting a new financial year (April 6) gives people the "perfect opportunity" to review their finances and switch from low-paying accounts.

Mr Walker added: "It's important savers take action and think about how they can make their hard-earned cash go as far as possible."

However, this opportunity could be limited, as the Government has confirmed it is reviewing the Cash ISA regime. This could be the last year to take full advantage of the existing £20,000 allowance.

Harriet Guevara, chief savings officer at Nottingham Building Society, said: "While no immediate changes were announced in the Spring Statement, the future of the Cash ISA allowance remains uncertain. We urge savers to use the start of this new tax year wisely and plan their Cash ISAs accordingly to make the most of this allowance while they still can."

While interest rates are falling following the Bank of England Base Rate cuts, some .

Here is a list of the ISA accounts offering the highest interest rates at the time of writing, according to Moneyfacts data.

Here are the top 10 easy access cash ISAs this week:

• Plum - 5.6% AER
• Trading 212 - 5.6% AER
• Moneybox - 5.56% AER / gross
• Chip - 5.55% AER / gross
• Tembo Money - 4.8% AER
• Monument Bank - 4.76% AER / 4.66% gross
• Moneybox - 4.57% AER / gross
• Kent Reliance - 4.56% AER/ gross
• Charter Savings Bank - 4.55% AER / 4.46% gross
• Buckinghamshire Building Society - 4.53% AER

Here are the top 10 Cash ISAs with a one-year fixed rate this week:

• UBL UK - 4.53% AER
• Charter Savings Bank - 4.51% AER / gross
• Vanquis Bank - 4.51% AER / gross
• Aldermore - 4.45% AER
• Kent Reliance - 4.41% AER / gross.
• United Trust Bank - 4.41% AER
• Castle Trust Bank - 4.4% AER
• Hodge Bank - 4.4% AER
• LHV Bank - 4.4% AER
• Paragon Bank - 4.4% AER.