LONDON, Oct 8 (Reuters) - German Chancellor Angela Merkel told Prime Minister Boris Johnson that a Brexit deal was "overwhelmingly unlikely" in a call on Tuesday unless Britain left Northern Ireland in the customs union, a British source said.

The source said if the call represented "a new established position, then it means a deal is essentially impossible not just now but ever".

"Merkel said that if Germany wanted to leave the EU they could do it no problem but the UK cannot leave without leaving Northern Ireland behind in a customs union and in full alignment forever," the source said, adding that Merkel told Johnson she thought the EU had a veto on Britain leaving the customs union.

(Reporting by Elizabeth Piper and William James)