Jozsef Szajer, a senior member of the ruling conservative Fidesz party, announced his resignation on Sunday.
Now the MEP has admitted attending a “private party” in Brussels on Friday and breaching Covid regulations.
He said he did not use any drugs, after media reports had suggested substances were involved.
“Police said an ecstasy pill was found. It’s not mine, I don’t know who placed it and how,” he said in a statement published on his website.
Twenty-five people were discovered by police in Brussels on Friday evening, including diplomats and an MEP who tried to escape, according to the La Dernière Heure.
“I deeply regret violating the Covid restrictions," Mr Szajer said in a statement on Tuesday. “It was irresponsible on my part.”
The MEP said he “drew the political and personal conclusions” for his actions with his resignation at the weekend.
With Belgium one of the worst-hit countries in Europe, the government has enforced a night curfew, tough quarantine rules and other measures to contain coronavirus.
Up to four people are allowed to meet outdoors, while you are allowed “close contact” with one other person inside under the restrictions.
After admitting breaking the coronavirus regulations, Mr Szajer, who belongs to Hungary’s far-right ruling Fidesz party, said:"I apologize to my family, to my colleagues, to my voters. I ask them to evaluate my misstep on the background of thirty years of devoted and hard work.
He said he told police he was a MEP, as he did not have any ID to show them, and received an official verbal warning before being taken home.
The Brussels prosecutor's office confirmed that police ended a lockdown party in a downtown Brussels flat on Friday evening after they were called because of a night-time disturbance - but did not confirm that it was a group sex party.
In a statement, the prosecutor's office later said police found about 20 people in the apartment, with two of these invoking diplomatic immunity after police checked their identities.
The prosecutor's office said a third man was arrested after a passer-by told police he tried to escape. Only identified as S.J., the man was unable to show ID and was escorted to his residence, where he produced a diplomatic passport.
"The man's hands were bloody. It is possible that he may have been injured while fleeing," the prosecutor's office said, adding that police found drugs in his backpack.
All those who attended the party have been reported for violating anti-Covid measures banning social gatherings.
In addition, a police report has been registered against S.J. for violation of the narcotics legislation.
Mr Szajer said he did not take any drugs and offered to take a drugs test, which police did not do.
Additional reporting by agencies
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