The British government says politicians might not attend the World Cup in Russia if Moscow is proven to be behind the unexplained illness of a former spy.

Sergei Skripal and his daughter are in critical condition after collapsing in the English city of Salisbury on Sunday.

A police tent covers the the spot in Salisbury, England, Tuesday, March 6, 2018, where former Russian spy double agent Sergei Skripal and his companion were found critically ill Sunday following exposure to an "unknown substance". 66-year old Skripal, and unidentified woman companion are being treated in hospital, after they were found unconscious on the park bench.(AP Photo/Frank Augstein)

Foreign Secretary Boris Johnson told the House of Commons there would be a "robust" British response if Russian involvement is proven, which could possibly impact on the participation in the World Cup.

In this image taken from security camera footage from PA shows a man and woman as they walk through an alleyway connecting a Zizzi's restaurant to a bench where former Russian double agent Sergei Skripal was found in Salisbury on Sunday March 4, 2018 The image is believed to be of interest to police. British counter-terror specialists offered expertise Tuesday to police in southern England as they sought to unravel the mystery of why a former Russian spy fell critically ill following exposure to an "unknown substance." Authorities maintained a cordon near the spot where former double agent Sergei Skripal and an unidentified woman collapsed Sunday in Salisbury. (AP Photo)

Johnson said "it will be very difficult to imagine that U.K. representation at that event will go ahead in the normal way."

The Foreign Office later clarified that the comment referred to dignitaries and officials, not the England football team, which is scheduled to compete at the tournament, which starts in June.