Wales's premier agricultural event - cancelled only three times before during both world wars and the 1947 fuel crisis - is staged annually at Llanelwedd near Builth Wells, mid Wales, and attracts around 250,000 visitors

The ground fell under emergency restrictions in March when the disease was discovered at a neighbouring farm.

And with 38 of the 67 cases in Wales occurring in Powys, pressure has increased on organisers to cancel the show.

Director Harry Fetherstonhaugh said it was the only practical option even though the society faced a loss of more than £500,000.

"It is the most horrific decision we've ever had to make. It was made with great reluctance," he said.

Mr Fetherstonhaugh added that postponment or a reduced event would be both impractical and costly.

The Royal Welsh Show is the jewel in the crown of Welsh agriculture, and the cancellation of this year's event will leave a massive void in the farming year that will be sadly missed

FUW President Bob Parry

The news has been welcomed by farming unions.

"I sympathise enormously with the society in having to come to this extremely difficult decision," said FUW President Bob Parry.

"The Royal Welsh Show is the jewel in the crown of Welsh agriculture, and the cancellation of this year's event will leave a massive void in the farming year that will be sadly missed.

"But I am sure that most farmers will agree that the Society has made the right decision in cancelling this year's event because of the enormous difficulties caused throughout the country by the disease."

The decision has been backed by the Campaign for the Protection of Rural Wales.

"We now need to forget about this year's event and start planning for next year to make sure the show and countryside survive," said director Merfyn Williams.

Even if the show - which pumps around £30m into the local economy - had gone ahead, many farmers would have been unlikely to risk bringing livestock in to show for fear of contamination.

The English and Scottish equivalents have already been cancelled, following consultation with farming authorities and others involved in the agricultural industry.

The Scottish event - cancelled on Wednesday- normally attracts 150,000 visitors and features.

The show is attended by 250,000 people

The Royal Highland and Agricultural Society of Scotland had hoped to stage a scaled down event without cloven-footed livestock like sheep, cattle and pigs.

Royal Welsh Agricultural Society met with the Welsh Assembly last month and issued a statement on 22 March saying a decision on the show would be made within weeks.

A deadline of 1 May was originally set.

Royal Welsh Chief Executive David Walters said last month that the delayed decision was taken to avoid acting "too hastily".

The show website had stated that alternatives being considered included postponing the event to a later date or staging it without cloven-hooved livestock classes for cattle, sheep, pigs or goats.