Jean-Claude Juncker was stumbling so much on Wednesday that world leaders including the Dutch Prime Minister forced to hold him up.
Footage from the summit shows the 63-year-old being held up as he makes his way down the stairs and stumbling as he attempts to walk.
Speaking on Friday, European Commission spokesman Margaritis Schinas confirmed Mr Juncker had ”suffered from a particularly painful attack of sciatica” which was “accompanied with cramps”.
When asked by a reporter if Mr Juncker was drunk, the European Commission spokesman said it is “more than tasteless that some press tried to make insulting headlines by exploiting President Juncker’s pain”.
Mr Schinas added: “I don’t think this is elegant, I don’t think this is fair.”
He later said that Mr Juncker “attended the dinner normally and then the pain disappeared” and went on to complete his full schedule of events on the following day.
The spokesman did not answer a question regarding whether the European Commission President had had an alcoholic beverage before dinner.
Mr Shinas said he was unaware whether Mr Juncker had received medical treatment following the event – but insisted the president takes medication to help ease the pain caused by sciatica.
He also said he “can confirm” that Mr Juncker’s condition was not preventing him from carrying out his role as European Commission president.
Mr Juncker has previously blamed his unsteadiness on sciatica, which can cause numbness and pain in the legs.
Mr Juncker was attending a gala at the Nato summit in Brussels attended by US President Donald Trump and First Lady Melania Trump.
Following the event, Dutch Prime Minister Mark Rutte said: “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.”
He added: “It is a problem in a sense that he sometimes has these pain attacks.”