What evidence has UEFA been looking at?

UEFA's control, ethics and disciplinary body will have considered the reports from the match referee, its match delegate and from 'spotters' in the crowd working for the Fare network, which works to stamp out discrimination and promote inclusion in the European game. It would also look at evidence from other sources, such as television footage and CCTV. Football Association chairman Greg Clarke said immediately after the game that security staff from his organisation would gather witness statements, and UEFA would also look at this sort of evidence.

What punishment could Bulgaria face?

Under Article 14 of UEFA's disciplinary regulations which relate to discrimination, the punishment may be to order the Bulgarian Football Union to play one match behind closed doors and issue a 50,000 euros fine (which equates to just over £43,500). However, there is provision under Article 14, 'where circumstances of the case require it' to impose additional disciplinary measures such as ground closure for multiple matches, forfeiture of a match, points deduction or disqualification from the competition.

What sort of punishments have been issued in the past?

The three-step UEFA anti-racism protocols were launched in a resolution at UEFA Congress in London in 2013. No game has yet been abandoned under the protocol. No team at club or national team level has yet had points deducted or been disqualified over an offence of racism.

On Monday, Serbia were ordered to play their next match behind closed doors with a further game to be added to that punishment in the event of further misbehaviour. Serbian fans were found guilty of racist behaviour during the Euro 2020 qualifier against Portugal in September.

One of the most severe punishments to date was the decision in July 2015 to impose a two-match stadium closure on the Croatian federation. That sanction was imposed because a Nazi swastika had been burned onto the pitch before a match against Italy in Split, which was already being played behind closed doors because of a previous offence.

The Montenegrin association was ordered to play a match behind closed doors and fined 20,000 euros after its supporters were found guilty of racially abusing England players during a qualifier in March. Tottenham defender Danny Rose said he was "lost for words" at the leniency of the sanction and added: "It's a bit shocking but there's not much I can do now. I just hope I don't ever have to play there again and we just have to move on now."

Players have faced more stiff punishments for racism-related offences – in August Ukrainian goalkeeper Kostyantyn Makhnovskyi, who was playing for Latvian side Ventspils, was banned for 10 matches.

Can there be an appeal against the decision?

All parties involved – the Bulgarian Football Union, the FA and UEFA, have the right to appeal against the decision made by the control, ethics and disciplinary body (CEDB).

What has been said?

UEFA president Aleksander Ceferin released a statement the day after the match urging the "football family" to "wage war on the racists" while FIFA boss Gianni Infantino pledged to extend any sanctions imposed by UEFA worldwide.

Anti-racism campaign group Kick It Out said: "There can be no more pitiful fines or short stadium bans.

"If UEFA care at all about tackling discrimination – and if the Equal Game campaign means anything – then points deductions and tournament expulsion must follow."