If your car has been caught in a flood, do not try to start it, photograph the damage and call your insurer straight away. If the insurer writes it off, it pays you the car's market value and takes the car. If you are not covered, or the car is not worth fixing, a licensed Authorised Treatment Facility can scrap it legally and issue a Certificate of Destruction.

Autumn and winter storms leave plenty of drivers in Britain looking at a car with a tide mark on the doors. Here is what to do next, step by step.

Should you start a car that has been in floodwater?

No. This is the most expensive mistake people make. Engines are built to compress air and fuel, not water. If water has got into the air intake and you turn the key, the pistons try to squash something that will not squash. This is called hydrolock, and it can bend connecting rods or crack the engine block in a second.

Water also shorts out electrics. Modern cars are packed with control units, sensors and wiring looms, many of them low down in the footwells or under the seats. Leave the ignition off and, if it is safe, disconnect the battery.

What should you do first after your car floods?

 - Stay safe. Never wade into moving floodwater to reach a car.

- Take photos and video showing how high the water came, inside and out.

- Do not start, drive or tow-start the car.

- Once it is safe, take out documents, valuables and anything that will rot.

- Contact your insurer as soon as you can and ask whether they will arrange recovery.

Keep a note of the date, time and location, and any receipts for taxis or hire cars. Your insurer may ask for them.

Does car insurance cover flood damage?

Comprehensive car insurance normally covers flood damage to your own car. Third party only and third party, fire and theft policies do not. Check your policy wording, because some insurers can reduce or refuse a claim if you drove into floodwater that was clearly visible.

Your excess still applies. If you have a courtesy car clause, read the small print, as some policies only provide one while your car is being repaired, not when it is written off.

When does a flood-damaged car become a write-off?

The insurer sends an assessor to compare the cost of repair with the car's value. Floods are hard on cars. Water gets into airbag sensors, engine management units and wiring, and salt water from coastal or tidal flooding speeds up corrosion. Faults can also appear weeks or months later, which makes repair a gamble.

If the car is written off, it is placed in one of four categories:

Category A

The car must be crushed. No parts can be reused.

Category B

The body shell must be crushed, but some parts can be salvaged. The car can never return to the road.

Category S

Structural damage that can be repaired. The car can go back on the road once it has been properly repaired.

Category N

Non-structural damage that can be repaired.

Badly flooded cars often end up in Category A or B, because water damage to electrics can affect safety systems such as airbags.

Can you keep a flood-damaged car after a write-off?

Only if it is Category S or N. Your insurer may let you buy it back, with the salvage value taken off your payout. Think hard before you do. Hidden corrosion and electrical gremlins are common in flooded cars, and the write-off marker stays on the car's history for good.

How do you scrap a flood-damaged car?

If you own the car outright and decide to scrap it, the process is the same as for any other vehicle. Get an instant scrap car quote by entering your reg and postcode. Be honest about the condition when you are asked, because quotes assume the car is complete, with four wheels and tyres.

Free collection needs a standard recovery truck to reach the car on solid ground. If your street or field is still waterlogged, you may need to wait until it dries out. On the day, have photo ID ready and remove any remaining belongings.

The Authorised Treatment Facility issues a Certificate of Destruction. You must tell DVLA yourself, using section 9 of the V5C or the DVLA website. Our guide on how to scrap my car covers the paperwork in full. Payment is made by bank transfer, because paying cash for scrap cars is illegal under the Scrap Metal Dealers Act 2013.

Is a flood-damaged car worth more as salvage than scrap?

Sometimes. A newer car where the water only reached the carpets may still interest a salvage buyer. Motorwise checks for salvage offers as well as scrap prices, so if a salvage buyer will pay more for your car, that's the price you'll see. Find out more about how to sell your car for salvage . Salvage quotes assume the car has never been written off, so tell the buyer if it has, as you may be offered a lower figure.

The bottom line

Do not start it, record everything and speak to your insurer first. If the car is beyond saving, scrap it through a licensed facility, get your Certificate of Destruction and notify DVLA so the car is no longer your responsibility.

Frequently asked questions

Will my insurer pay out if my car is written off after a flood?

If you have comprehensive cover, the insurer will normally pay the car's market value at the time of the loss, minus your excess, and take ownership of the car. Third party policies do not cover flood damage to your own vehicle.

Can I scrap a flood-damaged car without making an insurance claim?

Yes. If you own the car, you can scrap it through a licensed Authorised Treatment Facility, which issues a Certificate of Destruction. You then notify DVLA yourself.

Is free collection available for a flooded car?

Motorwise quotes include free scrap car collection, as long as a standard recovery truck can safely reach the car on solid ground. If the area is still flooded or waterlogged, collection may need to wait until it is accessible.