As many as 18,000 runners and spectators will descend on a weekend race event as part of trials to enable the safe return of crowds to mass gatherings.

London Marathon Events (LME) is organising the Reunion 10K – three separate races each involving 3,000 runners and 3,000 spectators – which will be held on April 24 and 25 at Hatfield Park in Hertfordshire.

The event is part of a Government scheme to gather scientific evidence on how venues can reopen without the need for social distancing.

Everyone attending – participants, spectators and staff – will be required to take a Covid-19 test before and after the event, and these are expected to be lateral flow tests.

LME said everyone will be asked if they have been vaccinated but this is not a requirement to attend.

Hugh Brasher, event director of LME, said each race will be set up “slightly differently”, telling the PA news agency: “We are doing three different ones to progressively learn from each one.

“So the protocols with each one, there will almost certainly be a slight difference on each one of them, and that is the detail that we’re currently working out with scientists and DCMS (Department for Digital, Culture, Media & Sport).”

Mr Brasher said the event, which is the biggest he is aware of in more than a year, is an important part of the process of returning safely to “some form of new normality”.

He said there has been a “huge drop off” in people doing sports events or activity, adding that events can give people a goal and a reason to run and do exercise.

“It gives them that motivation. And I think it’s been a really hard slog the last year for the whole of society.

“I think particularly people have felt this last lockdown to be incredibly difficult over the winter months, the darker nights.

“Now with the sun outside, last week having some beautiful weather, I think there’s a real optimism at the moment, and these events that the Government have announced are part of that optimism, and as I say we are absolutely delighted to be part of that,” he said.

Mr Brasher said the event is an “important milestone”, adding: “We absolutely believe that this is a key step to a lot of big events happening over the whole of the summer period.”