You wouldn’t think it given the embarrassing Northern Rock debacle, but chancellor Alistair Darling has been one of the main beneficiaries of the credit crunch.

National Savings & Investments, the government’s savings arm, expects to rake in £5.6 billion in the year to April, double its original forecast.

That should go some way to funding the growing shortfall caused by falling receipts from stamp duty given the housing-market downturn, and the dwindling sales of cash-strapped corporates.

NS&I’s pitch is simple: 100% security for your money because it is backed by the government, which is music to the ears of savers who queued round the block to pull their savings out of Northern Rock.

Except that their trust is misplaced. You can lose money with National Savings, in real terms at least, thanks to changes sneaked into its charter by Darling’s predecessor Gordon Brown.

In 2000, he scrapped one of the founding purposes of National Savings, which was to encourage people to save. NS&I’s sole aim now is to provide the government with the cheapest possible source of finance.

And that means the terms of the deals you get are generally dire — so much so that you can indeed lose money with NS&I once you factor in the effects of inflation.

Take the latest issue of NS&I’s Guaranteed Equity Bond (GEB). The government is ramming home the message that your money is safe, with full-page ads promoting the bond.

It promises a gross return linked to the FTSE 100, but with your money back in full even if the index has fallen by the end of the five-year term. Sounds tempting, especially with analysts invoking the Great Depression following falls in world markets.

The first big problem, though, is that you don’t actually get the full stock market return over the term — it’s capped at 75%. So if the market went up by 50% over five years, you’d get only 37.5%.

And then you get only capital growth, not dividends. With the FTSE yielding an average of 4.2%, that might not seem like a big deal — but it is. Dividends provide the lion’s share of total returns over the longer term.

The FTSE 100 index is lower now than it was 10 years ago thanks to the tech crash at the start of the decade and the credit crunch. Include dividends, however, and it is up 31%.

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