ISA providers are locked in fierce competition for savers during the so-called 'Golden Window', the four-week period straddling the 5 April tax year crossover.
Multiple banks and building societies are now offering inflation-beating returns exceeding 4 per cent as they vie for best buy status.
The annual scramble comes as savers face a rapidly closing window to make use of their full tax-free allowance before the current tax year concludes.
Andrew Hagger, Personal Finance Expert at Moneycomms.co.uk, said: "ISA rate rises are gathering pace as we approach the end of the 2025/26 tax year.
"This is being driven by a combination of providers jostling for best buy coverage plus increasing swap rates on the back of the Iran conflict."
Paragon Bank has responded to the competitive pressure by lifting rates across its entire fixed-rate Cash ISA portfolio.
The lender's one-year product now pays four per cent AER tax-free, up from 3.90 per cent, while its popular 15-month option has climbed to 4.33 per cent from 4.23 per cent.
Longer-term savers stand to benefit from more substantial increases, with both the two-year and three-year fixed-rate Cash ISAs rising to 4.20 per cent AER tax-free from 3.85 per cent.
The five-year deal has seen the largest jump, moving from four per cent to 4.35 per cent AER tax-free.
Paragon Bank increases interest rates across Cash ISA range
| GETTYParagon confirmed its refreshed ISA range will become available from March 24 through its online platform and postal applications.
Several other providers are also offering highly competitive rates during this period.
Tembo Money leads the easy access market at 4.55 per cent, though this includes a 1.15 per cent bonus for the first 12 months, while Atom Bank offers 4.25 per cent without restrictions.
For those seeking fixed-rate options, HampshireTrust Bank tops the one-year category at 4.31 per cent, followed closely by Investec Bank at 4.30 per cent and Vida Savings at 4.27 per cent.
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Tembo Money leads the easy access market at 4.55 per cent, though this includes a 1.15 per cent bonus for the first 12 months
| PAFurness Building Society currently offers the best two-year rate at 4.35 per cent, with HampshireTrust Bank and Tandem Bank close behind at 4.32 per cent and 4.31 per cent respectively.
James McCaffrey, spokesperson for TotallyMoney, said: "With the 'Golden Window' now well and truly open, we're seeing fresh 'best buys' come onto the market almost daily."
The 2026/27 tax year carries particular significance for savers under 65, as it represents their final opportunity to shelter £20,000 tax-free before the Cash ISA limit for this age group falls to £12,000 from 2027/28.
Andrew Wright, Head of Savings at Paragon Bank, said: "With the tax year end fast approaching, many savers are looking for ways to make the most of their ISA allowance.
The annual ISA allowance currently stands at £20,000 | GETTY "By increasing rates across our fixed-rate ISA range, including our popular 15-month ISA to a leading 4.33 per cent, we're giving savers greater choice and the opportunity to lock in competitive, tax-free returns across a range of maturity dates."
McCaffrey advised savers earning below four per cent to consider switching, noting that cash ISA transfers take no longer than 15 days.
He cautioned that early withdrawal penalties may apply to fixed-rate products, recommending savers maintain separate emergency funds in easy access accounts.