Buenos Aires, May 29 (RHC)-- Britain must be "fully out" of the EU customs union after Brexit in order to be a global trading nation, Foreign Secretary Boris Johnson said on Monday, piling further pressure on Prime Minister Theresa May.

"Now is our moment not to be less European -- we can do a great trade deal with the EU that will benefit both sides -- but to be truly global again," Johnson wrote in the Daily Telegraph.

Speaking after a long tour of South America, Johnson said countries in the region had been "emphatic" and told him: "If this is to work, we must come fully out of the EU customs union".

For Britain to be a "valid trading partner, then we must take back control -- as the PM has said -- of our tariff schedules, and do deals that are unhindered and uncomplicated," he said.

With just one month to go until a critical European Union summit, the British government is deeply divided over its plans for future trade ties with the bloc after Britain's withdrawal.

Johnson and some other ministers have advocated a clean cut with what is currently Britain's biggest trading partner that would enable it to sign free trade deals with third countries.  But moderates have put forward a plan whereby Britain would collect tariffs on behalf of the EU for goods heading to the bloc and setting its own, separate tariffs for goods that do not.

The EU has instead proposed a "backstop" solution to avoid a border in Ireland under which Northern Ireland would remain aligned with and de facto still under the bloc's rules, but Britain says that would effectively create a border between the province and the rest of the UK.

A senior EU official last week said: "We have to do away with the fantasy that there is an all-UK solution."  EU Brexit negotiator Michel Barnier on Saturday urged Britain to stop playing "hide and seek" in Brexit negotiations.