Plans for the South Wales Metro could be threatened if the UK leaves the European Union without a deal, the Welsh Government has warned.
Economy Secretary Ken Skates said it is likely officials in Whitehall dealing with a property transfer that is part of the project would be moved elsewhere.
He called for the transfer of track to the Welsh Government to be sped up.
A UK government spokesman said a final decision is likely this year.
As part of the project officials want to transfer some of the busy commuter lines that run north of Cardiff, including to Merthyr Tydfil and the Rhondda valleys, from Network Rail to Welsh Government.
That would give Welsh Government control of the lines' stations and their infrastructure, allowing it to upgrade the track for the tram-train rolling stock that has been promised.
There are plans for more frequent services and street trams in the capital by 2024.
According to a statement from Mr Skates the transfer is due to complete by 20 September this year, subject to a satisfactory business case.
Mr Skates said: "I have been made aware in recent days that the increased prospects of a 'no deal' Brexit now threaten this asset transfer process, with potentially far reaching impacts for the wider delivery of the South Wales Metro project.
"As a result of the need to handle a 'no deal' Brexit in Whitehall, it is likely that resources within the Department for Transport will be moved from ongoing business within the department and on to other duties."
Any reduction in resources and capacity available in UK government to support the transfer "jeopardises the deliverability of the South Wales Metro as contractually programmed", he said, adding there is also a risk to the availability of £159m in European funding.
Mr Skates said he has offered to discuss with the Secretary of State for Transport Chris Grayling "any practical steps that we could take to expedite the asset transfer".
A spokesman for the UK government's Department for Transport said: "We have been working closely with the Welsh Government on the divestment of the Core Valley Lines and we remain on track for the secretary of state to reach a final decision later this year."