LISBON (Reuters) - Portugal's UGT labour union said on Monday it will take part in a general strike called for June 27, making it the second time the two biggest unions would walk out together since the country's EU/IMF bailout in 2011.

The move will pile pressure on the center-right governing coalition, which took office in 2011 and whose popularity has dwindled after it enacted the largest tax increase in Portugal's modern history this year to meet stringent deficit targets under the bailout.

The more moderate UGT will join the more leftist CGTP, Portugal's largest union with around 750,000 members, which called the general strike on Friday.

"We say no to the dictatorship of the Troika - that is what we rebel against," Carlos Silva, the head of the 500,000 members-strong UGT, told a news conference on Monday.

The Troika is the European Commission, European Central Bank and the International Monetary Fund - Portugal's three lenders.

"We don't want or accept these austerity policies any longer," Silva said, accusing the government of dismantling the social welfare system.

The two umbrella unions will protest against public sector pay cuts, layoffs and other austerity measures imposed by the terms of the country's 78 billion euro bailout.

Silva said his union remained open to negotiation and stopped short of calling for the dismissal of the government as the CGTP did. Still, he said their position must be heeded.

"We are faced with a wall of intransigence. The government better understand that negotiating is not the same as imposing."

So far since the euro zone crisis erupted, strikes and protests have had little impact in Portugal, including the last general strike in November. They have all been largely peaceful when compared to those in Greece and in neighbouring Spain.

But Portuguese labour strife has intensified lately since April's rejection by the Constitutional Court of some austerity measures that forced the government to come up with alternative spending cuts and other unpopular steps, like raising the retirement age.

(Reporting by Daniel Alvarenga; Editing by Mark Heinrich)