Car had been standing in cold/wet weather for a week.
Started the car and engine fired up instantly. Idle was awful, however, revving between 300 - 2000 rpm. Problem disappeared very slowly after driving for half an hour. Has not returned since (=yesterday).

Any clue what it could have been?

Not ignition/HT I believe as it started okay.

Cheers!

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2011 Prius