The TUC has backed Money Marketing's Save our Mutuals campaign to change the law to help building societies fight off carpetbaggers.
The campaign has enjoyed a groundswell of support in recent weeks with the influential Treasury select committee backing Money Marketing's position, joining mortgage brokers and consumer groups.
The TUC has pledged its full support arguing that building societies use the economic advantage of not having shareholders to offer lower mortgage rates and a better return on investments than most high street banks.
The trade unions want the Government to enforce the Treasury select committee's recommendations to help mutuals deter carpetbaggers.
They also believe further conversions could bring branch closures, putting jobs at risk with some savers tempted by the short term gain of windfall payouts.
TUC general secretary John Monks says: "Building societies are currently extremely vulnerable to the predatory approaches of individuals.
"Greed and personal gain should not be allowed to take precedence over the hundreds of branches and thousands of jobs which could be at risk if carpetbaggers are to be allowed to continue to wreak havoc."
Britannia Staff Union general secretary David O'Dowd says: "Until now building societies have been easy prey to the carpetbaggers. One by one mutuals have lost their special status and those of us that are left are determined to hold on to it in the best interests of customers and employees.
"The day of the carpetbagger is coming to an end."