If President Donald Trump goes ahead and puts a steep tax on German goods crossing into the United States, Germany says it's ready to sue him through the World Trade Organization.

The comments come as German Chancellor Angela Merkel is preparing to meet with Trump at the White House early Friday 17 March for a series of meetings on trade and security issues.

Germany's economy minister Brigitte Zypries said in an interview with the German public broadcaster Deutschlandfunk early Friday that there are rules on import taxes that countries can impose.

Her comments represent a rebuke to Trump who — days before his inauguration in January — threatened he would impose a border tax of 35% on German cars and trucks made anywhere but America.

"I would tell BMW that if you are building a factory in Mexico and plan to sell cars to the USA, without a 35% tax, then you can forget that," Trump told the newspaper Bild on 16 January.

His comments followed news in early January that BMW intends to open new factory in Mexico in 2019. Despite Trump's warnings, BMW says that it intends to stick with the plans.

Zypries said that one response to that action would be for all countries to impose similar import taxes on American goods. But that would take a long time, she said. "The other option is we sue [Trump] at the WTO," Zypries continued, saying "it wouldn't be the first time that Mr. Trump has lost in the courts."

Her comments come as German leader Merkel prepares to meet with Trump, flanked by BMW CEO Harald Krueger.

Merkel has been preparing for the meeting for weeks and Kueger – and other German business executives joining the trip – are expected to highlight the hundreds of billions German industry has invested in US production and the roughly 700,000 Americans they employ. BMW's South Carolina plant employs about 8,000 people.

Donald Trump
President Donald Trump speaks to reporters aboard Air Force One as they approach Joint Base Andrews, Maryland on 15 March Jonathan Ernst/Reuters