The 40-year bonds sold are not among those bought by the Bank of England
The UK Treasury has failed to sell all its government bonds in an auction for the first time since 2002.
It wanted to sell £1.75bn of 40-year bonds, but investors only bid for £1.63bn of the debt, the Debt Management Office said.
Analysts said this may reflect concern over the state of the public finances as government borrowing surges.
But the Treasury said demand for gilts, as UK government bonds are called, remained "strong".
"It would be wrong to read anything into the results of one auction event, which depends on the gilt on offer, demand and market conditions on the day," a Treasury spokesman said.
The bid-to-cover ratio - how many bonds were offered versus how many were sold - was 0.93 at the auction, the first time in seven years it has been less than 1.0.
"It's a sign there isn't enough demand, which could be due to the amount of issuance that is expected to come, or people are worried about inflation," said Mohit Kumar, a strategist at Deutsche Bank.
The Consumer Prices Index, the benchmark measure of inflation, unexpectedly rose at an annual rate of 3.2% in February, from 3% a month earlier, because of the cost of imported goods.
Inflation erodes the value of bonds, which pay a fixed rate of interest over time.
The BBC's business editor Robert Peston said some analysts thought that gilt sales for 2010 may have to be as much as £200bn to fund the government's plans to revive the economy and meet its previous commitments.
"It's because the government has to borrow so much, that any such flop is unnerving," Mr Peston said.
The Bank of England this month began a policy of so-called quantitative easing, which involves effectively creating more money and using it to buy government debt.
It will inject a total of £75bn of new money into the economy. The goal is to push down the cost of borrowing.
Initially, the Bank said it will buy bonds maturing between five and 25 years, meaning that longer-dated gilts do not have a guaranteed buyer in the central bank.
"If an auction for sub-25-year debt failed, then I would be worried," Mr Kumar said.
Forty-year bonds are towards the longest end of government debt sold, meaning that buyers take the biggest risk to hold it.
Buyers for this kind of debt are usually pension funds and insurance companies that need them to pay annuities, which are regular incomes from a retiree's pension pot.