Passengers at Heathrow and Gatwick are facing long queues at passport control because of a fault with UK Border Force computers.
More top news
The mayor said a post-Brexit visa system was needed to maintain the supply of skilled migrant workers for London's economy.
Peter Barnett made hundreds of trips from his home in Oxfordshire to Marylebone station without paying the full fare.
Police say he may have attacked more children and urged them to come forward.