Rising air fares, energy prices and Vat all helped push inflation to a two-and-half year high last month.

But experts said the fragility of the economy was likely to ensure the Bank of England's monetary policy committee would continue to exercise caution over pushing up interest rates.

The consumer prices index (CPI) measure was 4.5% in April, up from 4% in March, said the Office for National Statistics.

CPI has not been at 4.5% since October 2008, after hitting 5.2% in September that year.

The increase was higher than anticipated and above the Government's 2% target, prompting Bank of England Governor Mervyn King to write to Chancellor George Osborne explaining the reasons for rise - the sixth quarter in which he has had to do so.

He pointed to January's increase in Vat to 20%, higher energy prices and rising import costs due to depreciation of the pound as reasons for the escalation.

"It is likely that had they not occurred, inflation would have been substantially lower, and probably below the target," he wrote.

"The monetary policy committee judges that attempting to bring inflation to the target quickly risks generating undesirable volatility in output."

Inflation has been 4% or more - double the official target - every month this year, and the Bank of England has forecast that it might rise to 5% before falling back. The April rate was the highest since September 2008 when it hit 5.2%.

"Continuing volatility in energy and commodity prices makes it difficult to be sure when inflation will return to the target," said Bank of England governor Mervyn King in a letter he is obliged to write to the Treasury when inflation is more than 1 point above or below target.

It was the sixth successive quarter in which Mr King has had to write such a letter.

The Bank has consistently blamed temporary price shocks, such as the boost to crude oil prices in recent months, for high inflation.

"Inflation is likely to rise further over the next few months, as increases in the price of energy are likely to raise petrol prices and make it more likely that there will be substantial increases in utility bills later in the year."

Northern Bank chief economist Angela McGowan said the Bank of England was in a difficult position as it weighed up the potential costs of a rate rise from their two-year 0.5% low.

"Unfortunately for the Bank of England, the economy remains too weak to impose a reactionary rate hike any time soon.

"Consumer sentiment is very negative, the output gap remains, business sentiment is deteriorating and exporters need all the support they can get.

"The governor has also recently pointed out that the UK debt burden is growing rapidly and an interest rate rise will only add to the cost of servicing that debt - putting further pressure on the economy."

The CBI's Northern Ireland director, Nigel Smyth, said businesses were feeling the pain of the hike, which he said was "not unexpected".

"Rising commodity prices, and in recent months energy prices in particular, are putting a strain on many sectors of the economy, with companies having to pass on price increases to consumers."

He said CBI surveys showed production costs for manufacturers had gone up "rapidly" in the first quarter of 2011, forcing one in three to increase their prices.

And there would be no easing up of inflationary pressures as firms expected costs and prices to go up sharply again over the next three months, largely due to higher commodity prices.

He added: "As the impact of the Vat rise falls away, inflation is expected to fall back closer to the Bank of England's 2% target rate next year. The CBI expects the Bank of England to introduce modest interest rate rises in the third quarter of 2011."

The most significant month-on-month contribution to the rise in the cost of living came from air fares, which the late Easter helped increase by 29%, the ONS said.

The rise in excise duties on alcohol and tobacco had a major impact on CPI as prices rose 5.3% between March and April.

The ONS said core inflation, which strips out volatile sectors such as food, energy, alcohol and tobacco, hit 3.7% - the highest since records began in 1997.

Andrew Goodwin, senior economic adviser to the Ernst -amp; Young ITEM Club, said the Bank was still likely to put off a rate hike until November. He said: "Though the MPC will derive no pleasure from seeing inflation being so high, there is nothing to suggest their central forecast - of inflation easing back to target towards the end of next year - will not play out so there's no reason to expect them to move earlier."