Juncker was stumbling and swaying to the extent that the presidents of Finland and Ukraine, among others, had to help keep the 63-year-old upright. Dutch Prime Minister Mark Rutte also stepped in to guide Juncker.
Footage from the summit also shows him struggling to make it up some steps and being held by the arm, at times by two other people.
At one point he can be seen looking worryingly precarious as he makes his way off a podium and nearly falling over backwards as he is led away by the arm.
Juncker has previously said he suffers from sciatica, a compression of the sciatic nerve, that sometimes affects his movements.
Portuguese Prime Minister Antonio Costa said Thursday that Juncker had complained of back pain.
'He doesn't have a serious health problem as far as I am aware of but he does have a back problem for some time,' Rutte said. 'It is a problem in a sense that he sometimes has these pain attacks.'
Juncker's office declined to comment, citing privacy concerns.
Leaders of European institutions were invited to join NATO leaders for a meeting and dinner on Wednesday night.
The NATO conference was marred by US President Donald Trump's suggestions that certain EU countries weren't contributing the 2% of GDP required towards the NATO budget.
The Daily Mail
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