The minimum legal tread depth is 1.6mm. And this needs to be a continuous band around the central three-quarters of the tyre — not just one groove.
Checking your tread depth is simple and there are two ways you can do this. The first — and most accurate way — is to use a tread depth gauge. Simply place the gauge into the main tread grooves at several points around the tyre. If it reads less than 1.6mm, you should replace your tyre as it’s illegal. Call into a local First Stop centre and they will happily perform the check for you.
If you haven’t got a tread depth gauge and you need to check your tyres immediately, you can do the 20p test.