Security company G4S is facing a new storm after failing to pay many of the staff they recruited for the Olympics and Paralympics.

Workers taken on by G4S, whose £235 million security contract for London 2012 was thrown into turmoil due to lack of staff training, have lodged complaints about losing hundreds of pounds a month from their pay packets.

Some staff, including workers from Glasgow, have claimed they have not been paid at all, while others allege they have been paid less than half of what they are owed for shifts worked over a two-month period. Claims have also been made that the company has refused to pay for training shifts.

One G4S employee, Kieran Dempsey, who worked at Hampden Park for the football preliminaries, said he is owed £500 from shifts worked in July and August and described the situation as "farcical". He claimed his girlfriend is due £800.

He added: "Some people are getting no money at all, but the most common problem is people are getting paid for selected shifts, but not for others. If they say they are going to pay you, you do expect to be paid."

Workers waiting for a response from G4S about pay issues have been getting an automated text message from the firm saying their query is being dealt with. Then, when complaining to G4S, they are told not to call but to email only. Some say they have waited a week for a response.

An online discussion forum used by Glasgow workers has been full of complaints about payroll for nearly two months.

One former employee claimed he was due £2100, but only received £1000. He said: "Anyone else want to gather the troops and march on Canary Wharf?"

The pay claims come as scores of workers nationwide have signed an open letter of complaint to the board of directors of G4S about "unprofessional, callous and unacceptable" treatment, including cuts to payments, not providing accommodation, insufficient training and revoking job offers at the last minute.

They have called on G4s to provide compensation to all concerned for the costs of attending interviews or training, including wages due or lost opportunities from the firm's sudden decision to revoke employment "costing individuals thousands of pounds in some cases".

G4S would not discuss the numbers of staff affected by the salary issue or how much was owed.

A spokesman said "We will ensure that staff will be paid what is due to them. As with any normal pay cycle, we will deal with any discrepancies to ensure all queries are resolved."