They are now paid according to the joint industry board agreement, which is set at about £13 an hour. Staff working on the nearby Terminal 5 project, which has been designated as a major project, earn more than £20 an hour.
Electricians have also complained that the pay rise they received in the new year was less than that allowed by the JIB agreement.
Frank Westerman, the union's regional organiser, has called the meeting for next Monday.
He said his members were unhappy with their pay rise. Some were also annoyed they were not receiving their full allocation of holiday pay under the agreement.
A source at one leading M&E contractor said that Amicus would be "very lucky" to get the scheme designated a major project.
He said: "The scheme is quite small in value, so Skanska and the client, which is the Home Office, will definitely dispute this."
He said the workers were probably angry about the pay disparity as the detention centre was close to T5, and was a high-profile site.
The detention centre is being built by Skanska. It will hold illegal immigrants prior to deportation.
A Skanska statement said: "Skanska is in talks with the trade contractor involved to ensure any issue there may be is dealt with."