Portugal is the latest nation to voice concern, as its civil aviation authority said that putting loaded guns aboard an aircraft could endanger it.

The use of sky marshals on transatlantic flights is controversial

In Britain, the pilots' union said that if the security risk to a flight is great enough to warrant an armed guard, the plane should not fly at all.

Denmark and Sweden have also rejected demands for armed guards on aircraft.

While the US has introduced new checks at home, its efforts to tighten security on incoming flights have met with strong opposition.

The director of Portugal's National Civil Aviation Institute, Joaquim Carvalho, told the AFP news agency: "We will not authorise loaded guns on Portuguese planes, therefore we will not allow armed guards."

He added: "If there is specific information about a particular flight which justifies having armed guards on board, what we would consider is cancelling the flight."

Pilots hostile

Elsewhere, the UK Government's announcement that it would put armed marshals on some flights "where appropriate" has drawn a hostile response from airlines and pilots.

British Airways has expressed concerns about having armed guards on board aircraft, while the holiday airline Thomas Cook has said it will ground any flights on which armed guards, known as sky marshals, are placed.

The British pilots' union, Balpa, said it disapproved of armed guards on flights and called for an emergency world summit of airline pilots to consider the demand for sky marshals.

In the meantime, the union is to have talks with the UK Government on Tuesday to discuss the issue.

Brazil has begun fingerprinting and photographing US citizens flying into its airports, apparently in retaliation for new security measures at US airports.

The Brazilian move came as the stringent new US regulations, affecting most tourists, were introduced.

Everyone entering the United States with a visa will now have fingerprints and photographs taken and scrutinised.