Scotland Yard is warning drivers to steer clear of the M25 when emergency services stage a mock crash tonight.
The motorway will close to traffic in both directions between junction 25 near Enfield and junction 27 near Epping from 10pm tonight (Saturday) until 6am tomorrow morning.
The "critical incident exercise" will involve emergency services from London, Essex and Hertfordshire.
The Metropolitan Police said diversions will be in place and clearly signposted.
A Met spokesman said: "This exercise is designed to test emergency services response to a serious collision that blocks the M25."
Highways England will carry out routine tests on equipment in Bell Common Tunnel near Epping and the Holmesdale Tunnel near Enfield during the eight-hour closure.
