US negotiators travel to EU headquarters in Brussels Monday to jump-start 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, or TTIP, 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 supra-national tribunals, which campaigners say would undermine sovereignty and favor corporations.

The historic drive to create a market of 850 million people, linking the 28-nation European Union and the US, 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.

The ambitious pact would be unique in history, analysts said.

It would not just slash the already low trade tariffs between the world's two top economies, but crucially it would also harmonize regulations to an unprecedented degree.

The Investor-State Dispute Settlement, or ISDS, allows firms to sue national governments through tribunals instead of national courts if they feel that local laws - such as health and safety regulations -  violate the trade deal and threaten their investments.

Opposition to ISDS became so intense that Malmstroem's predecessor Karel De Gucht, in order to move the talks forward, excluded it from the US-EU negotiations pending the outcome of a public consultation.

The EU received almost 150,000 replies - an EU record - and almost all were negative.

Anti-TTIP protesters also handed in a petition signed by 1.1 million people.

To make matters worse, some in the EU question Washington's true commitment to the deal, fearing it might be more interested in Asia.

"We have to ensure there is strong political commitment from both sides," said Business Europe's Santos.