The union's decision was a blow to efforts by Prime Minister Mohamed Ghannouchi to form the second national unity government in 10 days, following protests that forced Tunisia's authoritarian President Zine El Abidine Ben Ali into exile on Jan. 14.
UGTT's move was expected to increase tensions amid continued public protests meant to pressure officials to distance themselves from the former ruling party.
Five ministers dropped out of the current Cabinet soon after it was formed, including three UGTT members, to protest the continued presence of ministers from the former ruling party, the RCD. But the union backed Ghannouchi and two other ministers from the former regime to stay.
Earlier Thursday, Foreign Minister Kamel Morjane announced his resignation as authorities tried to quell continuing unrest by street protesters who want to oust other cronies of Ben Ali.
Protesters have complained bitterly about corruption, repression and the lack of jobs under Ben Ali's rule.
Morjane, a former defense minister under Ben Ali, was quoted by the official news agency TAP as saying he was resigning "in the interest of Tunisia" to help ensure that its "popular revolution" could lead to greater freedom. Like Ghannouchi, he had served under Ben Ali but remained in power amid the government's promised transition to democracy.
The caretaker government includes some former opposition leaders, but many top posts were retained by Ben Ali cronies. Demonstrators want those old-guard lawmakers out.