Hackers behind the leak of emails in which David Beckham allegedly complains about being snubbed for a knighthood reportedly demanded €1m from the sports agency representing him.

Police in Portugal, where Doyen Sports has an office, have been investigating the attempted blackmail for a year.

Beckham was not the explicit target of the hack, a source told the Daily Mirror, but was "caught in the crossfire".

A source said: "They approached by email and attempted to meet via another third party.

"They said we have all this information and you have to pay up. It was blackmail, pure and simple."

The leaked emails included one in which Beckham purportedly lambasts the honours committee which decides on recipients as "a bunch of c***s" and brands the honours system as "a f****** joke".

A representative of the former England captain dismissed the contents of the messages as "outdated material taken out of context from hacked and doctored private emails from a third-party server".

He said the material "gives a deliberately inaccurate picture".

The Daily Mail reported Beckham was on the verge of receiving a knighthood in the 2014 New Year's Honours list but HM Revenue and Customs placed a red flag on his nomination due to his involvement in an alleged tax avoidance scheme.

In response to his PR chief's email notifying him of the red flag, the newspaper said Beckham wrote: "The flag has no truth behind it as we didn't (do) nothing wrong everything is and was above board... C***s."

Katherine Jenkins with her partner Andrew Levitas, holding her OBE medal outside Buckingham Palace in 2014 (Getty)

Also in the leaked emails, Beckham is said to have questioned the OBE given to singer Katherine Jenkins, referring to a past admission she had taken drugs.

In response, a representative defended Jenkins, issuing a "fact sheet" listing her charity work and success in the music industry.

He said: "With regards to the taking of coke which Katherine has courageously publicly admitted with great honesty - this was when she was in her early 20s still a student - over a decade before she was awarded her OBE and started her recording career."

Other emails reportedly saw Beckham react negatively to a suggested donation to Unicef, of which he is a goodwill ambassador.

The charity said: "Unicef has become aware of media reports relating to 7: the David Beckham Fund for Unicef.

"Some reports relate to alleged private correspondence between our ambassador and other parties, which we have not seen and cannot comment upon."