England's clash with Bulgaria was halted twice due to racist abuse, with the first two steps of UEFA's three-step protocol being triggered.
An announcement was made over the loudspeaker at Stadion Vasil Levski demanding the racist chanting stop, which was made after referee Ivan Bebek paused the game.
The announcement, made by a UEFA delegate, made it clear that the match could be abandoned should the racism continue.
The delegate said: "Because of racist behaviour which is interfering with the game, the referee has indicated he may have to suspend the match. Please be in no doubt that the game may be suspended and abandoned if racist behaviour continues."
England players complained about racist abuse during the first-half in Sofia, with the message relayed Gareth Southgate on the sidelines, who spoke to the fourth official.
The first-step of UEFA's protocol was for the stadium announcement, with the second seeing both players leave the field momentarily while another announcement was made, which appeared to occur late in the first period.
Should the racist abuse continues, the game would be abandoned and members of the England squad had made it clear ahead of the match that they would be prepared to walk off if they suffered racist abuse.
England, with Liverpool's Jordan Henderson and Everton's Jordan Pickford, led the match by four goals to nil at half time.
