The militant national construction union and the Maritime Union of Australia have started historic negotiations to merge and form what they say will be the country's most powerful union.

The merger of the two left-wing unions represents a grasp for political survival by the embattled Construction, Forestry, Mining and Energy Union, which faces allegations of corruption before the royal commission into unions and a potential threat of deregistration.

MUA national secretary Paddy Crumlin has called Alcoa's late night raid an act of "industrial thuggery". Credit:Anthony Johnson

The merger is also likely to give the smaller Maritime Union of Australia more clout on the floor of the Australian Labor Party national conferences.

Former employment minister Eric Abetz said the merger was a dangerous "industrial relations cocktail combining the most corrupt union, the CFMEU with the most militant union, the MUA".