Staff of Heathrow Express have gone on strike, causing major disruptions, after a driver was sacked and a union representative was suspended.
Members of the Rail, Maritime and Transport (RMT) union have walked out following a dispute involving 250 staff.
Heathrow Express, has said it was disappointed the strike was going ahead and apologised to passengers.
An announcement on its website states: “We are disappointed the RMT Union has chosen to announce strike action against Heathrow Express.
“Heathrow Express apologises for any inconvenience to your journey and is committed to minimising any disruption.”
A second 24-hour strike is expected to take place on 11 March.