PRAGUE (Reuters) - Britain may have to leave the European Union's customs union when it quits the EU but it can maintain free trade with the bloc, British Foreign Secretary Boris Johnson was quoted as saying on Tuesday.

Johnson said in a newspaper interview that he believed Britain could have control over immigration while also keeping the benefits of access to the single market such as "passporting" rights for banks in Britain to do business in Europe.

"We want to leave the EU legal framework and treaties. We will probably have to leave the customs union, but that is a question that will be discussed," Johnson told Czech daily Hospodarske Noviny, in remarks published in Czech.

"I believe it can be done and at the same time maintain free trade and growing European economies."

He said "no" when asked whether he could imagine losing access to the single market.

Asked about Johnson's comments on the customs union, a spokeswoman for Prime Minister Theresa May said: "The foreign secretary reflected the government's position which is that a decision hasn’t been taken."

Johnson told the Czech paper that the free movement of people - which along with the movement of goods, capital and services, is one of the EU's "four freedoms" - was not a "founding principle" of the bloc, implying that it was not as fundamental as the single market.

"Everybody now has in their heads that every human being has some basic, God-given right to go and move wherever he wants. But it is not so. It never was a founding principle of the European Union. It is an utter myth."

German Chancellor Angela Merkel said on Tuesday that the EU could not separate the four freedoms to allow Britain to restrict immigration from the bloc while retaining tariff-free access to the market of close to 500 million people.

But she opened a door to discussions on the principle of the free movement of people, in a potentially significant development for Britain, where controlling immigration was a major issue for many of the 52 percent of Britons who backed leaving the EU.

Britain plans to trigger divorce proceedings with the EU by the end of March by invoking Article 50 of the EU treaty serving notice to its 27 partners of its decision to leave the bloc. The article sets a two-year period for exit talks.

(Reporting by Jan Lopatka; Additional reporting by Kylie McLellan in London; Editing by Hugh Lawson and Robin Pomeroy)