The US has issued a warning to its citizens of a potential Al Qaeda terrorist threat in Europe, leaving tourism chiefs across the continent concerned this could prompt a travel slump.

The travel alert is one grade below a formal warning not to visit Europe, but some experts said it could still hurt a fragile European economy already struggling to recover from the financial crisis.

The US State Department advised the hundreds of thousands of American citizens living or travelling in Europe to take more precautions about their personal security.

The travel alert noted in particular “the potential for terrorists to attack public transportation systems and other tourist infrastructure”.

“Current information suggests that al-Qaeda and affiliated organizations continue to plan terrorist attacks,” it said.

“European governments have taken action to guard against a terrorist attack and some have spoken publicly about the heightened threat conditions.”

But without a specific threat, it is expected that few US visitors will let the warning disrupt their existing travel plans.

“We live in New York. So in New York we think about these things all the time,” said Richard Mintzer, a 55-year-old American visiting Italy with his wife told the news agency AP. “I wouldn’t say we are particularly worried in Rome, no more than we would be at home, or anywhere in the Western world.”

United, Continental and Delta operated business as usual over the weekend without any cancellations or delays related to the new warning. The airlines said customers will be charged the usual penalty if they want to change itineraries.

However, the impact on travel could deepen if the threat leads to new, tighter security measures, and the biggest impact will be on those people that haven’t made their travel plans yet.

Foreign Office Warning

Meanwhile the UK’s Foreign Office began warning its travellers to France and Germany that the threat of terrorism in those countries is high.

Home secretary Theresa May said the threat of terrorism in the UK remains unchanged at “severe,” meaning an attack is highly likely.

Germany’s interior ministry said it saw no need to change its assessment of risks to the country and there were “still no concrete indications of imminent attacks” there. France’s interior minister said the threat of a terrorist attack is real but that the country is not raising its alert level.

A French official said Sunday that Italian police had arrested a Frenchman in Naples at the weekend. He was suspected of links to a network recruiting fighters for Afghanistan.

US intelligence officials believe Osama Bin Laden is behind the plan to attack several European cities.

Eight Germans and two British brothers are at the heart of an al-Qaeda-linked terror plot against European cities, but the plan is still in its early stages, with the suspects calling acquaintances in Europe to plan logistics, a Pakistani intelligence official said last week.