Striker Keegan, who later managed the national side, was "undoubtedly roughed up" with little provocation, and was left with bruises and blood pouring from his nose, the formerly secret files said.

Papers released by the National Archives contain a report from a clearly frustrated British diplomat in the former Yugoslavia about the events of June 2, 1974.

The unnamed official describes it as a "nasty incident" which began after a group of England players fooled around at a luggage transporter, with Keegan then ignoring police orders to move.

"He was dragged off for interrogation and undoubtedly roughed up, returning after 30 minutes with a bleeding nose, although not otherwise marked," the official wrote.

However, a Yugoslav investigation alleged "that K (Keegan) struck a policeman (and) concludes that it is only in the interests of Anglo-Yugoslav relations, international football etc that the police are not pressing well-justified charges against him".

The official Yugoslav attitude was "as to be expected", the official lamented.

"Official, total rejection of the possibility that any policeman could have acted as alleged, plus unofficial and entirely unquotable assurances that there will be some further enquiry behind the scenes."

The diplomat added: "I have some hope that police concerned will now be punished, although of course Yugoslavs would never admit that had happened.

"Per contra, if things blow up any further here, it is not totally impossible that police would be allowed to press charges in defence of Yugoslavia's reputation etc."

Belgrade insisted at the time that Keegan had been offensive and had struck an official.

The England players threatened to boycott the friendly match in protest, but ended up playing, with Keegan scoring the equaliser in a 2-2 draw.