Millions of people aged 65 and over are being urged to sign up for so-called pensioner bonds before Friday evening's deadline.
National Savings & Investments (NS&I) is taking on extra staff to cope with the expected increase in demand.
The bonds became the fastest-selling financial product in UK history when they launched in January this year.
The 65+ bonds offer exceptionally high savings rates compared to others in the market.
The one year product pays investors 2.8% interest before tax, and the three year product pays 4%.
The total amount available was originally £10bn, but that limit was later removed by the chancellor, George Osborne.
Instead the offer, external will expire at midnight on Friday 15th May.
The latest figures - released in March - show that 825,000 pensioners have bought the bonds; however a total of around 10m people could be eligible.
'Rush'
In comparison to the 2.8% interest rate on the one year bond, the next best available rate on the market is 1.9%, from online bank FirstSave.
While the four-year bond pays 4%, the next best rate is from Paragon Bank, which pays 2.5%.
"If you are eligible, and you haven't yet invested, I'd suggest going online as soon as possible," said Charlotte Nelson of Moneyfacts.co.uk.
"It's better than an ISA. I would rush now," she said.
However critics of the bonds have said they represent poor value for tax-payers, who are passing on an over-generous subsidy to pensioners.
NS&I has previously warned, external customers to be wary of scammers offering to sell the bonds.
In particular it warned about emails from someone called Kevin Archer, posing as someone from NS&I.
Investments in 65+ bonds are limited to £10,000 per person per product - in other words £20,000 for both the one and three-year products.
Applicants must be at least 65 , and have at least £500 to invest.
