The handover of the valleys railway lines that will form the core of the South Wales Metro could be delayed by Brexit, according to the Welsh Government.
Transport for Wales is due to take control of the tracks on all the routes that run to and from Cardiff Central via Queen Street.
Known as the Core Valleys Lines, they are due to be electrified by Keolis Amey, the consortium that also operates all Transport for Wales trains.
Network Rail is due to hand over the lines in September, provided that transfer is signed off by the UK Transport Secretary, Chris Grayling, in June.
The Welsh Government's Economy and Transport Minister, Ken Skates, is now warning that Brexit is putting that timetable at risk.
He claims that the Department for Transport is prioritising preparations for a no deal Brexit and might not be ready for the handover of the Core Valleys Lines.
Mr Skates says the handover of the lines, which carry more than half of all Transport for Wales train services, is critically important as any delay puts at risk the delivery of the South Wales Metro in line with Keolis Amey's contract.