Chancellor Angela Merkel has come up with what might be the best argument to convince sceptical Germans to back a proposed trade deal between the United States and the European Union: it would be good for beer.

Once finalized, the Transatlantic Trade and Investment Partnership (TTIP), would create the world's largest free trade area with 800 million people. But its opponents fear that it could water down consumer protections and give corporations more clout.

Speaking at a ceremony in the southern German state of Bavaria to mark the 500th anniversary of the country's beer purity law, Merkel said that the German beer business could be one of the winners of the trade deal.

TTIP opponents are planning a mass rally in Hanover on Saturday ahead of Sunday's visit by US President Barack Obama, where he is expected to promote the trade deal when he joins Merkel to open the northern German city's industrial trade fair. The US is for the first time the fair's partner nation.

"Obama and Merkel are coming: stop TTIP and CETA!," organizers said will be the motto of Saturday's protest. CETA refers to the Comprehensive Economic and Trade Agreement between the EU and Canada.

In an open letter to Obama ahead of his Hanover visit, Green EU lawmakers called it a "mistake" to speed up TTIP negotiations, fearing that "many European interests would be sacrificed."

The 13th round of TTIP talks is due to take place in New York next week. Negotiators on both sides of the Atlantic have expressed hope in concluding the talks under the Obama administration, but time is running short with US elections due in November.

The letter - signed by three EU lawmakers dealing with TTIP and US relations on behalf of the Greens - raises concern over standards to protect workers, consumers and small businesses.

But "medium-sized breweries in particular could improve their market opportunities" as a result of the US-EU trade deal, said Merkel, flanked by beer queen Marlene Speck and Bavaria's Economics Minister Ilse Aigner.

"He who does not have a beer, has nothing to drink," the chancellor added, quoting the 16th century German religious reformer Martin Luther after taking a sip of Bavaria's famed wheat beer.

But a survey of German voters released on Thursday showed what a hard sell the trade deal might be for the chancellor.

The survey, released by the Bertelsmann foundation, showed that only 17 per cent of Germans thought TTIP was a good idea.

The anti-free-trade alliance organizing Saturday's protest is made up of numerous associations, trade unions, artists and church groups, who fear that TTIP and CETA could threaten Germany’s environmental and legal standards.

A similar demonstration in Berlin in October attracted at least 150,000 people, with organizers claiming as many as 250,000 participants.

Concerns about the US trade deal also extend to many German businesses.

A survey released last month by the BVMW association representing Germany's small to medium-sized business sector, which has traditionally been the backbone of the nation's economy, found that 62 per cent of its members fear TTIP will have a negative impact on their industries.

However, considering the number of problems that have been piling up on Merkel's desk - such as the refugee crisis, relations with Russia, the Greek debt crisis and the terrorist threat - downing a beer on a sunny spring day in Bavaria would appear to be a welcome change for the chancellor. But the chancellor's agriculture minister, Christian Schmidt, later let it slip that Merkel's beer was in fact non-alcoholic.

In an interview with German's Bild newspaper, released overnight Friday, Obama described Merkel as a trusted and respectful partner on the world stage, and praised her political and moral leadership in the face of Europe's migration crisis.

He said that the most important message of his trip was that both the US and the world need a strong, prosperous and united Europe.

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