The FIFA president, in his first comment on the decision by Swedish referee Anders Frisk to retire after receiving death threats from Chelsea supporters, urged in a statement: "I call on all concerned parties that they show respect and fair play towards referees."

And Blatter, on the day that European football's refeeeing supremo Volker Roth described Jose Mourinho as "an enemy of football" after the Chelsea manager's criticism of Frisk, questioned the conduct of some "managers, directors and players".

"I am shocked by the recent verbal attacks aimed at referees. It is often this irrational behaviour that triggers the anger of supporters.

"I call upon all parties concerned to show respect for referees and for fair play," he said.

Blatter concluded by recalling that "those who attack referees are directly attacking the game of football that is their livelihood."

UEFA's director general Lars-Christer Olsson echoed Blatter's remarks.

"Everyone involved in football should think twice before making provocative statements which could be taken by others as an incitement to cause trouble," he said.

Frisk announced his retirement on Saturday following death threats from Chelsea supporters after their Champions League match against Barcelona at the Nou Camp last month.

Chelsea went on to win the last 16 clash with a 4-2 home victory after their 2-1 defeat in Spain on February 23 when Chelsea fans threatened to kill Frisk in messages by telephone, e-mail and the post.

"The game is just not worth all that," 42-year-old Frisk said.