A mock disaster which has taken two years to plan will test the response of emergency service teams across Sussex.

Details are being kept under wraps but the incident is expected to be one of the largest emergency planning exercises for years.

Police are warning the public to be aware of extra emergency service activity on Sunday, April 25, although they will only say the incident is taking place somewhere within the Mid Sussex corridor.

Previous mock exercises have included a serious petrol tanker crash in Southwick Tunnel, near Shoreham, a rail disaster at Brighton station and a plane crash at Gatwick.

Only a select few know exactly where the incident - codenamed Operation Woodchat - will take place. They will spend a day preparing the scene.

Mock incidents and drills are considered essential in training the emergency services.

The element of surprise is crucial in testing response times and the initiative of those involved.

The exercise will involve staff from emergency, health and local authorities as well as the Army, the Environment Agency and members of voluntary organisations.

Exercise director Superintendent Steve Voice, of Sussex Police's operations department, said: "To ensure the exercise provides a real test for the emergency services, we cannot publish too many details in advance.

"But the likelihood is people will notice a great deal of emergency service activity in various locations across central Sussex and on the coast during the day.

"There will also be many more emergency vehicles than normal on the roads.

"The important public message is people should not be alarmed.

"However, exercises of this kind are absolutely vital to ensuring that, if we were to be faced with a major civil contingency in Sussex, all the relevant services are ready and able to work together effectively to meet the challenge."