National Savings Investments (NSI) Index Linked Savings Certificates are among the most popular savings accounts on offer right now, as they give consumers the rare opportunity to keep their savings growing faster than inflation.

Demand for the certificates has helped boost inflows to the government-backed savings provider by £1bn in the three months to June, compared with the sums deposited in the same period last year. But their popularity has not been without problems.

Savers eager to take advantage of the reissued certificates – which pay 0.5 percentage points above the rise in the Retail Prices Index (RPI) – have reported difficulties in using the online application forms, and waiting times of up to an hour to speak to an assistant by phone.

Potential customers have also written to the FT to complain that the online process is not, in fact, a complete online experience. Applications made are not immediately confirmed, nor can accounts opened be viewed online.

Dug Falby, an e-commerce consultant, wanted to put money into NSI’s certificates but found the process frustrating. “The website moves between interactive and static pages and, in the end, I gave up,” he said. “It’s a less clean process than that of other online banking products and I think it should be available at the Post Office.”

NSI said that it had expected to see a high demand for the certificates when they were released and had hired extra help to man the phones.

Regarding the online application process, a spokesperson confirmed that the Index-Linked Certificates are not available to view online. Customers can complete the application process via a computer, entering their details and the account from which they wish to transfer the funds. But they must then wait for three days for a letter to be sent from NSI confirming their application and requesting a handwritten signature.

Savers cannot view their balances online but they can access an online calculator to work out how much interest they have earned so far. However, it is not, admitted the spokesperson, a “full” online experience.

NSI’s certificates are not being made available at the Post Office. Instead, the Post Office is offering its own inflation-pegged bond paying 1.5 percentage points over RPI on savings held for five years. However, as this is not an NSI product, the return is not tax free or guaranteed by the government.

Last week, the Post Office, which offers accounts run by Bank of Ireland, came under fire when a computer problem left millions of customers unable to access their money. The Post Office said the problem was resolved within the same day.

NSI enquiries can be made via 0500 007 007. Post Office 08457 22 33 44.