Mackay faces exit after Tan ultimatum
Cardiff City's Scottish manager Malky Mackay awaits kick-off of the English Premier League football match between Norwich City and Cardiff City at Carrow Road in Norwich, eastern England on October 26, 2013
Mackay was severely rebuked by Tan earlier this week after angering the club's supremo by asking to sign at least three players during the January transfer window.
The 41-year-old was told by Tan that he wouldn't get any new funds in the new year because he had already been handed a significant transfer kitty following Cardiff's promotion from the Championship last season.
Mackay responded by insisting he will not resign, but the BBC reported on Thursday that Tan sent the former Watford boss a letter by email on Monday, outlining his grievances at length.
Tan is said to have criticised Mackay's transfer signings, style of play and results before demanding he resign or be sacked.
A Cardiff spokesman refused to be drawn on the report, saying: "The club is not party to any letter sent from one person to another."
Mackay's position has been under scrutiny since Tan removed his head of recruitment Iain Moody in October amid suggestions of a summer transfer window overspend.
The Scot, whose contract as Cardiff's manager runs until 2016, is scheduled to hold his pre-match press conference on Friday ahead of Saturday's Premier League trip to Liverpool.