From the article:

Cause of fire confirmed

The cause of the huge blaze at London’s Luton Airport has been named as a diesel powered vehicle.

“We have no intelligence at this stage to suggest that this was anything other than an accidental fire that started in one of the vehicles that had not long arrived at the airport. It was not an EV,” Andrew Hopkinson, chief fire officer for Bedfordshire Fire and Rescue Service told reporters on Wednesday morning, The Mirror reported.

He added: “This was a diesel powered vehicle.”