New pensioner bonds paying an annual interest rate of up to 4% are now on sale but high demand has led to some website problems.
The government-backed lump sum investments are being offered through National Savings and Investments.
Market-leading rates and years of accounts offering low returns have led to expectations of rapid sales of these bonds.
Only those aged 65 and over are eligible to invest in the bonds.
Within an hour of the announcement of the bonds going on sale, there were problems with the NS&I website.
"We are currently experiencing high demand for our new 65+ Guaranteed Growth Bonds - customers may have to wait longer than normal to contact us. We're sorry for the inconvenience this has caused and are grateful to our customers for their patience. Our call centres are open 24/7," said a spokesman for NS&I.
The difficulties - which are affecting some customers intermittently - came despite a plea to pensioners to take their time over applications, and to apply over the internet.
"We expect these bonds to be on sale for months not weeks and would like to reassure savers that there is no need to rush to invest. We would also encourage savers to apply online. This should be the quickest and easiest way to invest and will provide immediate assurance that an application has been received," said Jane Platt, chief executive of NS&I, as the start of sales was announced.
Sylvia Waycot, editor of Moneyfacts financial information service, predicted that the bonds would actually be sold out in days.
The one-year bond pays an annual interest rate of 2.8% before tax, and the three-year bonds pays 4% before tax. Interest will be added on each anniversary after investment.
Financial advisers have pointed out that the best one-year bond on the open market was currently paying 1.85% interest and the best three-year bond was paying 2.5%.
"Now that a rise in interest rates looks even less likely this year, these new bonds look even more attractive and I expect them to sell like hotcakes," said Danny Cox, of investment firm Hargreaves Lansdown.
"Back in 2011, the popular NS&I index-linked certificates sold £5bn in four months before being closed. This year's bonds are a different product, but I would be highly surprised if the £10bn allocation lasts until the new tax year in April."
Tax will be deducted from the interest paid on these bonds, however non-taxpayers can claim this back from HM Revenue and Customs (HMRC).
Basic rate taxpayers must declare the interest if they complete a tax return. Higher and additional rate taxpayers need to declare the interest to HMRC and pay any further tax due.
Investment is limited to £10,000 in each bond, making a maximum of £20,000 per individual.
The creation of the bonds was first announced by Chancellor George Osborne in his Budget in March. He announced that they were on sale via Twitter.
Sam: 15th Jan 2015 14:38:00
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