Op Stack, as it's commonly known, is when lorries park or "stack" on the motorway when there is disruption to services at the crossing, including the Channel Tunnel or at the Port of Dover.

This can be due to bad weather, industrial action, fire or derailments in the tunnel, or amid increased security checks and border checks at either the port or at the tunnel.

All other traffic is diverted around the county.

Since its introduction, new phases have been added and a new design introduced. Here is everything you need to know:

How it works

Op Stack is used between junction 8 for Maidstone Services and junction 9 for Ashford.

If more space is needed, the closed section will extend to junction 11 at Westenhanger.

Police and other agencies separate freight into two queues on either side of the coastbound carriageway, one is for tunnel traffic and and one for port traffic. The middle lanes are kept clear for emergency vehicles.

Lorries are then released at the request of the Port of Dover and Channel Tunnel bosses and the lorries are moved down the queue in stages.

Sometimes this results in stretches of the motorway clear from time to time.

If junction 8 to junction 11 reaches capacity, Highways England will use Manston airfield to park Port of Dover freight and it is at this point the scheme officially gets called Operation Brock.

What to do when Op Stack is on

Freight - HGV drivers

When the scheme is in place, lorry drivers go to the police checkpoint at junction 8.

HGV drivers should make sure they have plenty of food and water and officers will direct drivers to the correct queue and engines should be turned off as drivers wait to be moved forward.

If junction 8 to junction 11 is full, Port of Dover freight will be directed to Manston airfield, but tunnel-bound freight will stay on the M20.

When Manston is in use drivers will see signs on the motorway directing them to the airfield in Thanet.

If drivers ignore the signs and go directly to the Port of Dover they will be turned away by officials.

How lorry drivers can get there

From the west drivers are urged to use M20 to junction 7 for Maidstone, where they will be diverted via the A249, M2 and A299 to get to Manston.

From the north (Dartford Crossing) lorry drivers are urged to use the A2, M2 and A299 to get to Manston.

From Manston lorry drivers should use A256 and A2 to get to the Port of Dover.

And freight carrying 'quick to market' goods will be sent directly to the Channel Tunnel or the Port of Dover.

These include livestock and other live loads (including live shellfish); fresh produce (including fish and meat); hazardous goods; freight/cargo for critical care (emergency medicines, equipment for critical care and any other material required to preserve life) and items critical for humanitarian need and welfare of the nation.

Lorry drivers carrying a quick to market goods should show their CMR note (proof of contract) to police on arrival at the Op Stack checkpoint.

Tourists and non-freight traffic

As the M20 is closed other drivers need to use the alternative routes which will be clearly signed posted.

Motorists should leave in plenty of time for their journey and if drivers are heading to the tunnel, they are urged to use the M20 and A20 and then follow the hollow circle diversion symbol.

Motorists and tourists heading to the port are urged to use the A2, M2, A2 route.

If drivers are already on the M20 when Op Stack is in place they will be diverted off at junction 7 via the A249, M2, A2 towards Dover.

Tourists and other drivers are also advised to check travel operators for advice before they leave and listen to traffic bulletins once on the move so you stay up-to-date. Eurotunnel can be reached by calling 08444 630000 and for The Port of Dover travellers should call 01304 240400.

In case of a breakdown, there will be recovery vehicles on standby to clear the route, but motorists are advised to check their cars carefully before setting off on a journey, especially tyres, lights, oil and fuel levels.

Tourists and motorists should also make sure they have food and water in case there are delays.

For the most up-to-date information

Check the Highways England website by clicking here - or follow @HighwaysSEAST on Twitter.

Visit the Kent Police Twitter page - www.twitter.com/kent_police

Check the KCC Highways website by clicking here or visit Eurotunnel by clicking here, or the Port of Dover here.

Avoid getting stuck on Kent's roads by keeping up-to-date on all of the latest traffic and travel news here or by following our live blog.

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