An outbreak of equine flu has forced the cancellation of all British horseracing on Thursday.

The British Horseracing Authority (BHA) made the decision on Wednesday night after the Animal Health Trust confirmed three positives tests from vaccinated horses in an active racing yard.

In a statement, the BHA reported horses from the infected yard raced on Wednesday at Ayr and Ludlow, adding identification of the virus in vaccinated animals presented a "cause for significant concern".

Ayr Racecourse said in a statement: "We were informed by the BHA this morning that three vaccinated horses in an active racing yard tested positive for Equine Influenza.

"Horses from the yard raced at both Ayr and Ludlow yesterday although they were not among those who tested positive.

"In the UK horses are widely vaccinated. Like other stakeholders, Ayr will continue to work closely with the BHA on this matter."

Scottish horse trainer Lucinda Russell said she would testing all her horses over the next 48 hours.

She said: "It's very serious. We're fortunate to have two yards and so last night we isolated the horses that had been to the Ayr races in one of them.

"We'll be testing all of our horses for equine flu over the next 48 hours. We're also taking the temperatures of every horse.

"Thankfully we have a very high bio security here to ensure no contamination between the two yards.

"But it's an issue that needs to be taken very seriously. We can't allow the spread of it among the racing population and the general horse population.

"This is not a fatal disease but it reduces the immunity of the horses."

Thursday's cancellations come less than five weeks before the start of this year's Cheltenham Festival - the annual highlight of the National Hunt calendar.

It is not yet known how long the current shut-down of racing may have to last - but inevitably for thousands of racing followers, and of course those directly involved in the industry, there will be uncomfortable echoes of the foot-and-mouth crises of 1967 and 2001.

On each occasion, the racing calendar was affected for two months - and in 2001, the Cheltenham Festival was abandoned.

A further update on the possible continued extent of disruption is expected from the BHA - with a packed weekend of Cheltenham trials and other big races scheduled at Musselburgh, Newbury, Warwick and in Ireland.