Glasgow Subway staff have been told they may have to sell tickets for a replacement bus service outside stations when they shut for a month during an upgrade.

Many could also be asked to travel with late night revellers and check their tickets while work is carried out on the ramps and turnouts which allow trains to travel from a depot at ground level to the underground tunnels.

The Evening Times has learned that the service will stop on June 24 for “a minimum of four weeks” – sparking fears that the multi-million pounds modernisation may take longer than expected.

And workers who are protected from the public by plastic screens could now be “nose-to-nose” with late night travellers outside shut stations, or on replacement buses.

A letter to staff from a senior manager at Strathclyde Partnership for Transport (SPT), which oversees the Subway, gives staff two options.

One option will see more than 150 workers, including drivers, “undertake temporary duties” including “regulating customers”.

A source at the Subway said this could mean selling tickets for a replacement bus service and travelling from station to station checking tickets.

The letter from the Subway’s operations manager said shifts would finish as late as 11pm on weekdays and 9.30pm at weekends.

If workers select this option they will “continue to receive full contractual pay,” according to the senior manager.

The second option could see workers cleaning closed stations or taking part in training sessions.

The letter states: “Although there will be no customers to serve, there are a range of duties in current job descriptions which can be carried out including stations, car parks, light cleaning duties etc. as well as additional training and competence assessments which could be carried out.”

Staff have been issued with a form and told to tick option one or option two then return it to bosses “as soon as possible”.

The letter added: “The sooner we have clarity about staff preferences for the above option, the sooner we can start advising you about what your duties will be during closure.”

A source at the Subway said: “They’re closing it on the 24th of June and it’s a shambles. They’ve got no idea what they are doing.

“Initially they wanted all of their staff to go on replacement buses and sell tickets but women were apprehensive about doing that late at night. Now they’re saying they will set up some sort of kiosk outside stations which will be shut.

“Some staff will have to sit on the buses and basically regulate the bus and make sure everyone has a ticket. Or stand at stations and point people to bus stops.

“That would mean we’d be nose-to-nose with all sorts of people during the hours the subway operates.

“They say it will be a minimum of four weeks but nothing done in the Subway runs to schedule so I won’t be surprised if it’s six weeks or more.”

It is understood the trade union which represents staff considered balloting them for industrial action but many members have already indicated which option they prefer.

A source at the Rail, Maritime and Transport workers' union (RMT) said: “As it stands, SPT has put forward two options to staff.

“The majority of them have now sent in replies so we can’t justify having a mass meeting to discuss any action.”

An SPT spokeswoman said: “SPT has been working very closely with staff and union representatives to deal with the issues raised, and to ensure we are focussed on continuing to deliver excellent customer service, particularly during this period.”