National Savings and Investments insisted the sales of the Bonds would continue for months, rather than weeks, and stressed there was no need to rush.
Yet, there have been reports this morning that the NS&I website has crashed, with would be customers taking to Twitter to complain. NS&I said it had been experiencing high demand and thanked users for their patience.
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Sylvia Waycot, editor at Moneyfacts.co.uk, warned retirees expecting to be able to purchase one of the bonds that they must be quick or face disappointment.
Up to £10 billion has been made available to cover both the one and three year terms, enabling the sales to last for months, NS&I said.
But Ms Waycot said: "This bond is going to be gone in days if not sooner. If you want the chance to get hold of one, then waste no time in applying. Disregard any notion of postal applications and don't get stuck in a long phone queue as it really is a case of quickest gets the deal. My advice is to apply online."
Jane Platt, chief executive, NS&I, said: "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."
Ms Waycot claimed that the on sale date had been much less heralded than the announcement about the creation of the bond in the first place.
She said: "Many of the people eligible for the newly launched 65+ Guaranteed Growth Bonds must be feeling pretty miffed by the quietness of the actual launch, which is in stark contrast to the very loud original announcement made by Chancellor George Osborne.
"While the bond is clearly marketed for any pensioner quick enough to apply, only those who signed up to the NS&I newsletter will have received the heads-up."
The bonds will pay 2.8% gross on a one year fixed rate bond and 4% gross on a three year bond and are only open to those aged 65 and over.
Investment will be limited to £10,000 in each bond meaning an individual can invest up to £20,000 across the two bonds.
Pensioners investing the maximum of £10,000 in a three-year bond will earn £1,248 before tax. According to Moneyfacts the average rates for one and three year bonds (excluding 65+ Guaranteed Growth Bonds) today stood at 1.43% for 1 Year Fixed Bond and 2.03% 3 Year Fixed Bond.
The new bonds, unveiled in the March 2014 Budget, are lump sum investments designed to be held for a whole term but can be cashed in early with a penalty equivalent to 90 days' interest.

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