Southgate's words came in the wake of Raheem Sterling being the target of alleged abuse at Wembley last Saturday. The PA news agency understands that the 24-year-old, a key figure in the fight against racism, was the focus of discriminatory language during the first half.

A steward heard the individual in the Bulgaria section of the ground and they were ejected from Wembley and handed to the police, before being released with no further action.

"Yes, it is a concern," Southgate said about the return game. "It's something that we've already planned. We've already planned what our schedule looks like and we're going to discuss it with the players before we go, because we're aware that there is history there and we want to make sure that we're all prepared for what might happen and how we want to respond."

But the BFS has reacted angrily to Southgate's comments.

"The president of the Bulgarian Football Union Borislav Mihaylov sent an official letter to the UEFA General Secretary Mr. Theodore Theodoridis and the chairman of the English FA Mr. Greg Clarke expressing his indignation and disappointment following the public comments by Mr. Gareth Southgate and the suggestions that the English players will be subject to racial abuse by the Bulgarian supporters in the upcoming European qualifier in Sofia on October 14th," a statement read.

"The BFU views such behaviour as groundless, inappropriate and unnecessary prior to the important and exciting football game that Bulgarian fans have been looking to in the past year. We have not received any official information regarding discriminatory language used by a Bulgarian supporter at Wembley Stadium.

"Even if this were the case, however, we believe that generalisations and the creation of needless tension by official members of the English team is absolutely unacceptable and in contradiction with the spirit of mutual respect and fair play, the core values of both UEFA and the BFU."

Next month's qualifier will be England's first trip to Bulgaria since September 2011, when a 3-0 win was overshadowed by racist abuse in Sofia.

Ashley Young was subjected to monkey chants during that game, but the Bulgarian Football Union (BFS) only received a 40,000 euros (£34,000) fine by UEFA for "discriminatory" chanting and for the lighting and throwing of fireworks.

The Vasil Levski National Stadium will already be partially closed for England's latest visit due to the racist behaviour of their supporters in the 2-1 loss in the Czech Republic in June.

The BFS is required to block off at least 5,000 seats for the visit of Southgate's men and display a banner with the wording '#EqualGame'.

Bulgaria's return fixture against the Czechs is also due to be played at a partially-closed ground due to racist behaviour in the 3-2 home loss to Kosovo in their other June fixture.