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Anti-royal visit protesters have infiltrated security at the venue for Ireland’s Grand National horse race – and daubed a ‘F**k The Queen’ slogan in front of the grandstand.

Security has been stepped up at Fairyhouse racecourse in Ratoath, County Meath, after the vitriolic slogan was burnt into the ground in front of the main viewing area ahead of Monday’s big race.

The Irish Independent reports that the slogan only came to light on Sunday afternoon, just before racing began, after weed killer was used to write the words.

The phrase ‘F**K The Queen’ was painted near the finishing line and only became visible from a height to people in the stands at lunchtime on Sunday as the four day festival of racing was about to get underway.

Workers immediately moved to block out the slogan with cut grass and security was stepped up at the venue, linked with the 1916 Rising as many British Army officers attended Fairyhouse while Rebels took control of Dublin’s GPO.

Peter Roe, acting manager at the course, told the Independent of his shock at the anti-Royal action but said the vast majority of racegoers were unaware of it.

“We are not aware of who did this and the weedkiller may have been poured on the grass three to four days ago,” he said.

“We’ve done our best to sort it out. We’ll have extra security on. We’ve had security for the last two nights.

“This didn’t happen on Saturday, this happened a couple of days ago. This only came to light on Sunday.

“We will up the security and police presence as the National is very important. It is a national institution.”

Police are already investigating the incident, less than a month before Queen Elizabeth flies into Dublin for the first Royal visit in the history of the State.

Agriculture Minister Simon Coveney was critical of those behind the protest.

“I think it is sad that people are still so stuck in the past, that they might try to damage the reputation of the Irish horse racing industry by trying to get across what are totally outdated views that don’t represent the views of anybody really left in Ireland,” said Minister Coveney.

Local politician and Junior Minister Shane McEntee said: “Such an act is pure nonsense and people will not put up with that sort of behaviour.”

Read more: Queen a legitimate target, says top dissident Republican

Read more: McGuinness warns IRA dissidents they are in a fool’s paradise