A source in the Electricians and Plumbers Independent Union said it was close to finalising details of a complaint that it intended to make to the Monopolies and Mergers Commission if the deal went through.
The source added that plans were being drawn up by the unions' national officers to persuade MPs to back the complaint.
Unions are concerned that workers across London would be overly reliant on a merged agency for work, and that this could inhibit them from making complaints about conditions and welfare.
The EPIU source said: "The agency would have a virtual monopoly – there would be nothing to stop them blacklisting workers that they see as troublesome."
There would be nothing to stop them blacklisting workers that they see as troublesome
A senior source at the AEEU said general secretary Ken Jackson had been unhappy with ESCA Services for some time. He said the agency had not been consistently making money and that sealed bids for it had been received over the past few months.
ESCA Services and BMS refused to comment and the ECA was unavailable for comment.
The joint secretaries of wage body BATJIC agreed that the craftsman hourly rate will increase from £6.70 to £7.30 and an adult general operative's hourly rate will increase to £5.65 from £5.42.