Forty mostly Polish construction workers at a Metronet site at King's Cross Station in London walked off the job on Friday 25 May in a dispute over late payments.
It is understood the workers refused to return to work following a row with subcontractor Hennelly's.
According to GMB organiser Tom Kelly, who was also in talks with main contractor Balfour Beatty about resolving the dispute, Hennelly's had not paid the workers for four weeks.
The total sum at the centre of the argument is believed to be in the region of £120,000.
Kelly said, 'We are appalled that companies can behave like this, especially where public funds are involved. We'd have hoped that the vetting process would have been more thorough.'
One worker said that Hennelly's had been involved in a similar dispute at Heathrow's Terminal 5 project, where workers recently walked out after not being paid.
Hennelly’s say the dispute is resolved.