Jean-Claude Juncker will undergo surgery in November to treat an aneurysm, members of his inner circle told AFP on Thursday.

The 64-year-old former Luxembourg premier had been due to retire on Friday from EU's executive arm.

But that has been postponed until at least December, because his successor Ursula von der Leyen has failed to win parliamentary approval for her commission.

Juncker "will undergo surgery on November 11 to treat an aneurysm," his spokeswoman said.

According to AFP's sources, Juncker is suffering from an aortic aneurysm in his abdomen, or an AAA in medical parlance.

Juncker's duties will be covered by commission vice president Frans Timmermans and the president expects to be back at work in time to attend the handover to Von der Leyen.

If untreated an AAA can rupture with potentially fatal consequences. It was not immediately clear how serious Juncker's case was.

In August, Juncker cut short a vacation to undergo urgent surgery to have his gallbladder removed, and he has often shown signs of discomfort at public events.

Juncker has held Brussels' top job for the past five years, despite recurring problems with back pain, but his commission has been winding down business in recent months.

He is not expected to make any big political decisions in the coming weeks, as the current commission is now just overseeing everyday business while von der Leyen's team prepares to take over.

Von der Leyen, the former German defence minister, had hoped to make a fresh start from this week. But the European Parliament rejected three of her nominees for commission posts, leaving her in limbo until new ones are put forward and confirmed.