Jozsef Szajer was serving his fourth term as an MEP. /AFP

A conservative Hungarian MEP and close ally of Prime Minister Viktor Orban apologised Tuesday after Belgian police caught him at a lockdown-busting party that local press described as an orgy.

Jozsef Szajer, who helped write Hungary's constitution, resigned over the weekend for what he then said were "personal reasons", but admitted on Tuesday he had received a police caution.

Brussels prosecutors told AFP that 20 men were caught at a city centre party on Friday and fined €250 each. Local press called it a "sex party".

Szajer, 59, made no reference to sexual activity and denied taking drugs, but he apologised to his family for breaking COVID-19 restrictions.

"I was present," he said, in a statement distributed by his conservative political group.

"After the police asked for my identity -- since I did not have ID on me -- I declared that I was an MEP. The police continued the process and finally issued an official verbal warning and transported me home."

"I deeply regret violating the COVID restrictions, it was irresponsible on my part. I am ready to stand for the fine that occurs," he said.

Szajer has been elected four times to the Hungarian parliament between 1990 and 2002, and four times to the European Parliament since 2004.

His party, Orban's Fidesz, called him "the best-known and most-recognized Hungarian member of the European Parliament".

On Saturday, Szajer resigned as an MEP effective December 31, but said it "has nothing to do with the current, animated policy debate taking place on the European level".

This was a reference to the deadlock between Hungary and Brussels over its controversial veto along with Poland of the bloc’s long-term budget and coronavirus recovery fund.