US negotiators travel to EU headquarters in Brussels Monday to jumpstart talks on the world's biggest-ever free trade deal, which after nearly two years remain bogged down by public opposition.

The future of the Transatlantic Trade and Investment Pact, orTTIP, is in doubt in the face of bitter opposition by activists and mixed signals from key governments, including Europe's biggest economy Germany.

"This is the dirtiest trade deal in Europe's history," a new video posted by the anti-TTIP group Corporate Europe said.

Particularly controversial is a plan to let companies have legal disputes with governments heard by supranational tribunals, which campaigners say would undermine national sovereignty and favour corporations.

The historic drive to create a market of850 million people, linking the 28-nation European Union and the United States, began 20 months ago and on the eve of the eighth round of talks many believe the process is at a make-or-break stage.

The four days of talks starting Monday will be the first since the new European Commission led by Jean-Claude Juncker took office in November, with the outspoken Swede Cecilia Malmstroem charged to salvage the talks as the new trade commissioner.