An MEP from Hungary's ruling party, which is hostile to LGBT rights, has quit after he was caught attending what Belgian media describe as a gay sex party in Brussels on Friday.
Jozsef Szajer, a top member of Prime Minister Viktor Orban's Fidesz party, was stopped by police after reports of a house party violating lockdown rules.
Prosecutors say he was found with drugs as he tried to flee the gathering, reportedly shinning down a drainpipe.
He apologised for "a personal failing".
On Friday he had pleaded parliamentary immunity but he is now under investigation - for both non-compliance with measures relating to the Covid-19 pandemic and violation of drug laws.
The European Parliament said the immunity rule only extended to an MEP's official duties, not to their private life.
What happened on Friday?
At around 21:00 (20:00 GMT), prosecutors say, police were alerted by neighbours who complained about noise and potential lockdown breaches in an apartment located on Rue des Pierres in the centre of the Belgian capital.
Inside the flat, police found about 20 people, two of whom - not including Mr Szajer - invoked diplomatic immunity.
Most of those present were men, some of whom were naked when police burst in, Belga news agency reports.
They were cautioned for having broken the lockdown rules.
A passer-by who told police he had seen a man descending a drainpipe was able to identify him.
The man's hands were bloody and it is possible he was injured while fleeing.
"Narcotics were found in his backpack," a statement by prosecutors says. "The man was unable to produce any identity documents. He was escorted to his place of residence where he identified himself as S. J. (1961) by means of a diplomatic passport."
Criminal proceedings may only be brought if parliamentary immunity is waived, the statement adds.
What had Mr Szajer to say about the incident?
The BBC's Nick Thorpe in Budapest says Mr Szajer's sudden resignation as an MEP on Sunday surprised his political friends and foes alike. He had blamed "increasing mental strain" for the decision.
In a statement on Tuesday, Mr Szajer admitted he had attended the party on Friday but did not describe it.
"I didn't use drugs, I told the police on the spot I was willing to undergo an official test, but they didn't do one," he said.
"The police said an ecstasy pill was found. It was not mine, I don't know who planted it or how. I made a statement to the police about this."
"I am sorry that I broke the rules of assembly," he added, "this was irresponsible on my part, and I will take the sanctions that come with it."
Belgium's De Standaard daily says the usual fine for breaking the Covid rules on assembly is €250 (£224).
Jozsef Szajer's resignation, followed by his confession that he took part in what media have described as a "gay orgy", is a serious blow for Fidesz, which has governed Hungary for the past decade.
Mr Szajer was one of the founders of Fidesz in March 1988, and remains a close confidant of Prime Minister Orban. He has been an MEP for 16 years, for the past 11 as chief whip and vice-president of the European People's Party.
He has played a key role in keeping Fidesz inside the conservative political grouping, despite fierce criticism. He is married to Tunde Hando, former president of the National Judiciary Office, now a Constitutional Court judge.
The sex scandal is particularly embarrassing for a party which campaigns for traditional family values and recently proposed a law to ban gay adoption.