Frances O'Grady, general secretary of the TUC, will join labour movement leaders at a two-day meeting studying the injustices suffered by workers around the world.
She will say: "The Catholic Church and trade unions both understand that much more can be achieved together than alone.
"We share values of community, dignity and social solidarity, values that bind us together as workers, citizens and human beings.
"Together, we can improve working lives and put dignity for working people ahead of market forces and freedom of capital.
"We can build a popular alliance for economic justice, in Britain and around the world.
"So speak out and support the courageous workers who stand up against injustice.
"Call on your congregation to join a union, and encourage Catholic employers to do the right thing and recognise trade unions."
