Do not start the car or turn on the ignition as this will activate the fuel pump. Call the RAC or AA or similar immediately. They deal with this a lot and will need to drain the tank.
Putting petrol in your diesel car will damage your fuel pump if not dealt with immediately. In diesel cars, the main fuel pump uses the diesel fuel flowing through it for lubrication. However, when you put petrol fuel in, the mix of petrol in diesel acts as a solvent, reducing lubrication and quickly causing damage to the pump as the metal parts come into contact and rub together. Other parts of the fuel system which might not be compatible with petrol, can also be damaged.
Switching on your car’s ignition will circulate this mix of petrol in diesel through your fuel system, causing further contamination to other components..
Ultimately, if left for too long this could mean that the entire fuel system requires costly repairs, or replacing completely!