Though aged only 58, Juncker has been in office for 18 years and in government for 30. He is best known in Europe for a tumultuous eight year stint as head of the eurozone finance ministers group, which ended in January.
In a rare moment of political drama in tiny Luxembourg, Juncker addresses parliament from 2pm (1200 GMT) over a report alleging that the Luxembourg secret service he is supposed to oversee - the SREL - indulged in a spate of misconduct from 2003 to 2009 that included illegal phone-taps, corruption, and even a dodgy car dealership.
A vote endorsing the report would undermine Juncker's coalition, made up of his Christian Social People's Party and junior partner, the Socialist party.
Socialist leader Alex Bodry said earlier Wednesday that he expected "a strong gesture" from Juncker but feared he had "chosen another option."
"When one thinks one is infallible it's hard to admit to making mistakes," he told the daily Luxemburger Wort.
The opposition says it is ready to call for a censure motion against Juncker.
The report was put together by a parliamentary committee after a Luxembourg weekly last year published a secretly-taped conversation in 2007 between Juncker and the then head of SREL, Marco Mille.
In the recorded conversation, Mille revealed that his staff had also secretly taped a conversation with Luxembourg's Grand Duke and that the sovereign was in regular contact with Britain's MI6.
But the parliamentary inquiry set up in the furore that ensued uncovered more dirt: the existence of 13,000 secret files on people and businesses, illegal wire-taps on business leaders and even a murky private dealership in luxury cars.
"The commission of inquiry concludes that the prime minister, as head of the intelligence service, not only had no control over his service but also too often omitted to inform the parliamentary control committee or the judiciary of its irregularities, aberrations and illegalities," the report said.
Juncker last month survived the duchy's first confidence vote in 150 years over a separate scandal. The last confidence motion in Luxembourg was in 1848 and led to the fall of the government.
Voice of Russia, AFP