They aren't say what constitutes a mass gathering yet, but will publish the guidelines "next week"
I hope by that they mean Monday at the latest as there isn't time to fanny about!
From the news article:
"The Whitehall source said the government, in dealing with coronavirus, has been "concerned" about the burden that large events put on public services including the NHS and police.
The source added: "We have drafted emergency legislation to give the government the powers it needs to deal with coronavirus, including powers to stop mass gatherings and compensate organisations. We will publish this legislation next week."