Chelsea fans were seen holding up an image of the so-called “SS death skull” in Budapest last Thursday, a revelation that compounds allegations of antisemitism at the team’s Europa League tie with Vidi.

Photographs posted on social media showed club supporters holding up a Northern Ireland flag that was defaced with elements including the Totenkopf skull and cross symbol, used as an insignia by the Nazi SS paramilitary organisation and other German units during the Second World War.

Complaints were lodged with European football’s governing body Uefa after Chelsea supporters were reportedly heard using an antisemitic term in chants during the match against Vidi.

In a chant that is familiar to Chelsea regulars, the words "Barcelona, Real Madrid, Tottenham are a bunch of Yids... Yiddos" could be heard from the Groupama Arena away end during Thursday's match.

The flag carrying the SS death skull symbol, which also references the Chelsea Headhunters, a hooligans' group with roots in the late 1960s, is not thought to have been waved during the game.

The Observer reported the fans carrying it were believed to have remained in central Budapest away from the stadium during the stadium, and therefore out of the club’s jurisdiction.

Any fan caught attempting to smuggle such a banner into the stadium would have risked incurring a ban from Chelsea.

In a statement on Friday after reports of antisemitic chanting emerged, a Chelsea spokesperson questioned the “brain-power” of some of its fans.

The spokesperson said: “Antisemitism and any other kind of race-related or religious hatred is abhorrent to this club and the overwhelming majority of our fans. It has no place at Chelsea or in any of our communities.

"We have stated this loud and clear on many occasions from the owner, the board, coaches and players.

“Any individuals that can’t summon the brain-power to comprehend this simple message and are found to have shamed the club by used using antisemitism or racist words or actions will face the strongest possible action from the club.”

Uefa is expected to rule on the complaints in a matter of days.