The Glasgow Subway will have the first completely unstaffed trains in the UK.

Strathclyde Partnership for Transport (SPT) said the new trains due to be introduced in 2020 will only be staffed at the busiest times.

They will be tested near Ibrox Stadium later this year and will initially be operated by drivers before going fully autonomous in 2021.

SPT senior director Charles Hoskins said: "We will always have staff in the system but it is not our plan to have a member of staff on every train."

Similar systems are already in place in Paris, Barcelona and Copenhagen.

The Unite union's Pat McIlvogue said busy matchdays at Ibrox could cause problems.

"SPT's future vision involves a Subway system which is ticketless, with no counter staff, no staff on platforms and no staff on trains," he said.

"If that is to be the case, Unite has to raise the issue of what happens when something goes wrong?"

Other UK railways including Docklands Light Railway in London have driverless trains crewed by staff. Glasgow Subway will be the first to have completely unstaffed trains.

SPT said it was still considering "how it might best utilise frontline staff" after the new fleet is introduced.

Tunnels and signalling on the subway's six-mile loop are being upgraded to enable trains to run every three minutes, compared to four minutes at present.

Trains will also be able to run on the route - popularly known as the Clockwork Orange - up to every two minutes before and after football matches.

They are also expected to operate later at night and on Sundays, boosting the number of passengers each year, which currently stands at around 13 million.

The 17 new trains will be open-plan with no doors between their three carriages to maximise space and provide room for wheelchair users.

They replace 13 trains introduced when the Subway was last modernised in 1980.