Tottenham Hotspur fans are at the centre of a controversy over the use of the word "Yid", after the FA warned that fans could face prosecution if they use terms offensive to Jewish people.
However, David Cameron and the Tottenham Hotspur manager, Andre Villas-Boas, have contradicted that message, arguing that fans should not be prosecuted.
Spurs supporters argue that the chants are part of their tradition.
Comedian David Baddiel, who has campaigned against the use of the word, insists the word is a race hate word and should be given the same level of protection as other racist terms.
ITV News' UK editor Lucy Manning reports:
More top news
The threat level has been raised to critical by MI5's Counter-Terrorism Analysis Centre.
Prime Minister Theresa May announced the decision from Downing Street.
A man, apparently armed with a small axe and a large stick, was detained close to the event and police cleared the area as a precaution.