Boris Johnson and Portuguese Foreign Affairs Minister Augusto Santos Silva | Andre Kosters/EPA | Andre Kosters/EPA Boris Johnson appeals to Portugal to help unblock Brexit talks ‘If one side is involved in some very difficult European negotiations, it is the duty of one side to come to the aid of the other,’ the UK foreign secretary says.

LISBON — British Foreign Secretary Boris Johnson appealed to the terms of a 630-year-old treaty Friday to solicit the aid of England’s oldest ally in his Brexit negotiation travails.

"In the Treaty of Windsor, if you read it very carefully, somewhere I'm sure, there is a clause that says ‘if one side is involved in some very difficult European negotiations, it is the duty of one side to come to the aid of the other,'" Johnson told reporters after talks with his Portuguese counterpart, Augusto Santos Silva.

The 1386 treaty sealing the world’s oldest diplomatic alliance was just one of the historical ties that Johnson recalled as part of a charm offensive in the Portuguese capital, one of several visits he’s making to European capitals in an attempt to unblock the Brexit talks. He also referred to the Duke of Wellington’s role in chasing Napoleon’s troops out of Portugal in the early 19th century, and mentioned that British fictional super spy James Bond had been “conceived, at least intellectually, in Estoril,” a plush Lisbon beach suburb.

More interestingly for his hosts, Johnson paid homage to the 400,000 Portuguese residents making a “wonderful contribution to our lives, culture and economy” in the U.K. and said it was “vital for both sides that their rights are protected.”

The rights of its citizens in Britain has been the top priority for the Portuguese government.

Johnson also said the old alliance would survive and be modernized after Britain’s departure from the EU, and stressed the two countries' similar outlook as “freedom loving, Atlanticist countries.”

Santos Silva expressed hope that “Brexit will not be the collapse of the alliance, but on the contrary will increase our responsibility to improve our bilateral relations.”

In an interview earlier this month, Santos Silva told POLITICO that despite the solid relations between Britain and Portugal, Lisbon had no plans to play a mediating role in negotiations, saying he respected the decision of the EU27 to have a joint negotiation led by Michel Barnier.