The ruling body quickly enforced a six-day shutdown of racing in Britain, but the IHRB confirmed racing would continue in Ireland - with all runners from Britain not be permitted to race until further notice.

However, with the British Horseracing Authority due to decide late tonight whether racing can resume in Britain on Wednesday, the IHRB said in a statement: "Following on from the update issued on Friday, 9th February 2019, the Irish Horseracing Regulatory Board (IHRB) today announced that runners from Great Britain will be permitted to race in Ireland with immediate effect provided that the horses comply with the IHRB requirements."

Dr Lynn Hillyer, chief veterinary officer at the IHRB, added: "They are fine (to run in Ireland), provided they can fill the requirements that we set out in our release on Friday night, which said that horses need to have received a vaccine for equine influenza which contains Clade 1 virus within eight weeks of their race.

"As long as they can fulfil that requirement, they are fine. Obviously, by definition, they will only be coming from yards which aren't under restriction from the BHA.

"Having had the opportunity to consider things over the weekend, the board have made the decision that we are able to support that movement - which is obviously good news for everybody.

"There was a discussion whether to wait until after the BHA's announcement tonight - but we decided at this time the two things are independent.

"The critical thing is they have to have had the correct vaccination within the eight weeks preceding the run."

The British Horseracing Authority is to make a decision late tonight on whether racing can resume on Wednesday following the recent outbreak of equine influenza.

In the latest update this afternoon, the BHA said no decision should be expected "before 10.30pm at the earliest", which will allow the industry veterinary committee - which includes representation from the BHA, Animal Health Trust (AHT), British Equine Veterinary Association (BEVA), National Trainers Federation (NTF), Racehorse Owners Association (ROA), Racecourse Association (RCA) Thoroughbred Breeders Association (TBA) and independent expertise - "to have access to the most recent evidence".

A statement read: "Analysis is ongoing at the AHT, in line with a prioritisation plan based on the potential exposure of horses to a horse that has tested positive (for example at the fixture at Ayr on February 6), or to a horse from a yard returning a positive test.

"Swabs will continue to be collected and tested from other horses, but some have been fast-tracked to help inform this evening's decision-making process.

"Separately, a specific plan is being formed as to what the clearance process will be for individual yards to return to racing. The details of this plan will be confirmed as part of the decision making process for any return to racing.

"Should the decision be taken to return to racing on Wednesday, declarations for any fixtures staged will be at 10am on Tuesday. The BHA would provide further updates regarding the declaration process for any fixtures staged on Thursday and beyond."

The news appeared positive after 720 initial tests for the virus came back negative on Saturday.

However, although the BHA has revealed approximately 700 further tests carried out on Sunday also returned no other positive samples, there was a blow late on Sunday evening after the BHA announced that four vaccinated horses at the yard of trainer Simon Crisford had been found to have the virus.

Crisford says there is "no obvious connection" between the horses that have tested positive and their stablemate who ran at a potential risk fixture last week.

Crisford was one of the 174 stables to be placed in lockdown because he had a runner at Newcastle last Tuesday - after which trainer Rebecca Menzies, who had also had runners at the meeting, reported a "suspicious" case.

Menzies' horses have subsequently returned negative results for equine flu, while Crisford confirmed his Newcastle runner, Sajanjl, is also clear of the virus.

In a statement, Crisford said: "None of the four horses that have returned positive tests for equine influenza displayed any clinical signs of respiratory illness, including nasal discharge and elevated temperatures, prior to the mandatory swabbing that was undertaken last Friday, February 8.

"The swabbing occurred following Sajanjl's race at Newcastle last Tuesday, February 5, and she has tested negative. There is no obvious connection between Sajanjl and the four identified horses.

"All horses at Kremlin House Stables, totalling 92 boxes, undergo a strict vaccination check and programme on their arrival.

"All four identified horses have been vaccinated within the last six months along with the rest of the yard and in line with vaccination protocol."