The Union of Construction Allied Trades and Technicians (UCATT) said staff at Green Hill Construction were told it was closing on 11 April.
Workers were given no notice period and were not consulted, the union said.
The firm, which employed over 150 people, built houses for housing associations.
Green Hill was set up in 2006 to build affordable homes for social landlords and had worked for associations including Seren and Cardiff Community Housing Association.
In January 2013 it announced it was creating 20 new jobs after receiving Â£146,650 from the Welsh government's economic growth fund.
Dave Gunter, regional organiser for UCATT, said the company had showed no signs of being in trouble before its closure announcement.
"The first we knew about it was when our members called saying they had been told there was no work for them," he said.
"It was obviously a sad day for them."
Mr Gunter said workers had been advised by the firm to claim outstanding payments from the Redundancy Payments Service, but the union had been unable to contact the company to confirm that.
BBC Wales understands the future of the company will be resolved next week.
Green Hill Construction could not be contacted for a comment.
The Welsh government said it would work with the company to see if jobs could be secured.
A spokeswoman said: "The company received financial support from the Welsh government and we always seek to recover public funds if and when possible to do so.
"Our first priority is to assist the company and its staff at this difficult time."
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