The stadium is expecting sell-out for the Wales-England match

Professor Michael Clarke of the Centre for Defence Studies at King's College, says it would be logical for the next attacks to be outside London.

Venues where crowds gather are likely to be at risk, he added. When asked, he agreed the stadium could be among them.

Police stress there is nothing to suggest a specific threat in Wales.

However, despite there being no intelligence to suggest terrorists are planning an attack in the area, the force has urged the public to remain vigilant.

Cardiff plays host to the Wales-England World Cup Qualifier a week on Saturday, with thousands of football fans expected to watch the game in city bars as well as in the stadium.

Professor Michael Clarke said while London was the obvious place for the first attack, terrorists are now likely to strike in towns and cities away from the British capital.

More than 70,000 people can fit in the ground on match day

"London is now a very difficult place to target for any potential terrorists groups so it's logical that somewhere out of London might be attractive to terrorists thinking about it," he said.

"What they are looking for is where the public congregate, targets which have a symbolic value in some way, because terrorists try to 'send a message' and also places where they feel security might be stretched because of a big event.

"Any big sporting venue could be a target. The objective now is to spread fear and so even the possibility of a terrorist attack in big venues can cause lots of headaches and keep the level of anxiety relatively high."

'Tried and tested plans'

But Paul Sergeant, General Manager at the Millennium Stadium, said Prof Clarke's claims were "nothing new" and the like any large venue, the stadium had taken measures to sep up security.

"Plans are revised on an event-by-event basis at the stadium as they are at every venue around the country," he told BBC Radio Wales.

He said one problem all such venues faced was sections of the media determined to "stitch up" venues by trying to sneak in a bag to test security arrangements.

"That diverts so much of people's time that it's completely uncalled for," he said.

South Wales Police Detective Chief Superintendent Geoff Cooper said: "A lot of planning has gone into the operations within south Wales, and clearly we've learnt lessons.

"We've been involved in the de-briefs that have taken place post the atrocities in London. So we constantly review what we have.

"There are tried and tested plans that have been rehearsed."