Firms have been warned not to use Brexit as an excuse to cut jobs and workers' rights amid warnings that a "shadow" was hanging over workplaces in the aftermath of the EU vote.
Len McCluskey, general secretary of Unite, said productive factories like Ford's engine plant in Bridgend, south Wales, have had their future thrown into doubt following the referendum result.
"We are ready to work with employers to overcome any genuine problems that may exist, but we are not prepared to see Brexit used as a smokescreen for further disinvestment from Britain. Öut of the EU must not mean out of work," he said at the TUC Congress in Brighton.
Mr McCluskey also called for an end to the "shameful racist backlash" on migrant workers in recent months.
"We need to say loud and clear - migrant workers in Britain are our brothers and sisters and we stand shoulder-to-shoulder with them against the racists. It is greedy bosses who are to blame for driving down wages, not migrant workers."
The Unite leader called on unions to "pick up the pieces" following the referendum result to protect workers' rights.