Racing will not resume in Britain until Wednesday at the earliest following an outbreak of equine influenza.

More than 50 yards, including those of champion trainers Nicky Henderson and Paul Nicholls, have been placed into lockdown by the British Horseracing Authority (BHA) for testing.

The lockdown applies to British-based trainers who had runners at Ayr or Ludlow on Wednesday or Wolverhampton on Monday. All horses will be tested today and must be passed clear of the virus before they can run again.

A decision about when racing may resume will take place on Monday.

“This is a very serious matter and the repercussions could be bad for everyone in the game and therefore these are extremely worrying times,” Henderson said.

“We are involved because we had runners at Ludlow yesterday so we’re not allowed to run anything tomorrow or make any entries, but the two horses that ran yesterday were taken out of the yard straight away at 7am this morning well away from Seven Barrows and are isolated on their own away from everyone else.”

The Authority’s decision meant a second day without racing being staged and the abandonment of four meetings scheduled for today at Kempton, Bangor, Southwell and Newcastle but, more worryingly for the sport, the potential loss of prestige cards due to take place this weekend unless they can be restaged at a later date.

Late on Wednesday, the BHA elected to abandon the four meetings scheduled for yesterday after being informed by the Animal Health Trust of three confirmed cases of equine influenza from vaccinated horses in a racing yard.

Vaccination of all racehorses is mandatory in both Britain and Ireland but that cannot always be effective against a new strain of the virus.

The yard in question was confirmed as that of Donald McCain. The trainer had runners at both Ayr and Ludlow and, given the speed at which the virus can spread, potentially exposing a significant number of horses from yards across Britain and in Ireland, the BHA felt the need for swift action.

Gordon Elliott, one of the leading jumps trainers in Ireland, had five runners at Ayr but was able to place those horses in an isolation stable rather than returning to his main yard. Other Irish trainers with runners at the meeting had the time to adopt similar precautions which meant that the fixture at Thurles yesterday afternoon took place as planned, although British-based runners will be barred from racing in Ireland for the present.

The loss of high-profile meetings due to take place at both Newbury and Warwick on Saturday brings the concern of the impact that would have upon trainers’ plans for given prep runs to horses ahead of next month’s Cheltenham Festival.

The meeting scheduled for Wolverhampton on Saturday evening had already been cancelled as McCain had a runner there on Monday, and the racecourse stables will undergo a deep cleanse operation before another meeting can be held at the course, with the same process expected to be employed at Ayr and Ludlow.

Officials at Musselburgh racecourse now face possibly losing the track’s most valuable fixture.

Musselburgh’s fixture on Sunday was only announced by the BHA on Monday in response to the loss of the two-day Cheltenham Trials meeting and the fixture at Wetherby the previous weekend due to frost. The card comprised a number of the races from the original Musselburgh meeting with £160,000 in prize money.

The BHA said: “A plan will be constructed for the rescheduling of key races – and those which may provide important opportunities for horses to run – which are lost during this period.”