Excited savers clamoured for "granny bonds" which went on sale today, but the stellar demand has caused the National Savings and Investments website to crash.
"We are currently experiencing high demand for our 65+ Guaranteed Growth Bonds - customers may have to wait longer than normal to contact us," a spokesman for NS&I told the BBC.
"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," he said.
Investors over the age of 65 are being offered a one-year bond with 2.8 per cent interest, and a three-year bond with four per cent interest. They can put no more than £10,000 in each bond, which amounts to £20,000 per individual.
The rates - which are before tax - will give investors much better returns than equivalent products being offered by banks and building societies.
"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," Jane Platt, chief executive of NS&I had said as the sales were announced today.
"This should be the quickest and easiest way to invest and will provide immediate assurance that an application has been received," she said.
Experts expect over-65s to pour cash into the bonds, which offer market leading rates, meaning they could be unavailable in as little as four weeks' time.