SUBWAY passengers have been facing major delays today after bosses were forced to shut down two stations because of a tunnel fault.

Strathclyde Partnership for Transport said it had identified a problem between Cowcaddens and Buchanan Street, the network's busiest station. Both were closed during the morning rush-hour.

Early reports that there had been a partial tunnel collapse between the two stations were ruled out by SPT.

However, the transport authority said engineers were carrying out checks on the tunnel.

Buchanan Street reopened at noon, but Cowcaddens is expected to remain shut until tomorrow at least.

SPT said a problem had been identified during checks of the 113-year-old subway which are being carried out as part of a £300million upgrade.

It said the subway was still running as normal but passengers would not be able to get on or off at Cowcaddens.

A shuttle bus service was being run between St Enoch and St George's Cross.

An SPT spokeswoman said: "We have a localised engineering problem between Cowcaddens and Buchanan Street. There is no tunnel collapse but we are checking out the tunnel.

"The only thing was to shut down the two stations. Buchanan Street has since reopened but Cowcaddens will be closed, most likely for the rest of the day.

"We are probably being over cautious but safety of our passengers is paramount.

"We can still run trains. Everything else is running as normal.

"We are running a shuttle bus service between St Enoch and St George's Cross so we can drop passengers off.

"Part of the upgrade is carrying out checks to see what needs enhancement."

The modernisation of the subway, which is the third oldest in the world, will include driverless trains and signalling.

The transport authority has promised to keep the underground operating throughout all refurbishment works and say the majority of the work should be finished by 2020.

It will be the third major revamp in the subway's history, after electrification in the 1930s and the 1977-1980 shutdown.

That latter project saw the subway almost entirely rebuilt.