The former Scotland international was appointed manager in June 2011 and led the Welsh club to promotion from the Championship seven months ago, but has been criticised by the businessman for saying he hoped to recruit three new players in the January transfer window.
Herald Sport understands that Tan outlined his grievances, including transfer budgets, signings, results and style of play, in an email to the Scot on Monday.
Controversy has never been far from Tan since he took over in May 2010. Last season, he changed the club's colours from blue to red and added a red dragon to the club's crest then relieved head of recruitment Iain Moody of his duties and hired 23-year-old Alisher Apsalyamov, who was at work experience at the club and is a friend of Tan's son, as his replacement. He is also reported to have tried to influence the tactics of the first team during matches.
One of Tan's concerns surround a perceived overspend of the transfer budget agreed between Mackay, chief executive Simon Lim and Moody.
Cardiff broke their transfer record three times over the summer, signing striker Andreas Cornelius for £7.5m, defender Steven Caulker for £8m and midfielder Gary Medel for £11m. Tan claims the total budget was £35m and that £50m was spent including add-ons. Mackay and Moody have denied those claims, insisting the £35m was solely for transfer fees and that Lim and Tan had no complaints about that rising with add-ons.
Brendan Rodgers, the Liverpool manager, was one of the first to react to the news last night, saying: "My only conclusion when I look from the outside is you've got a business guy operating the club who obviously knows nothing about football. I find it astonishing, what he's [Mackay] had to go through. To see him being questioned on what he has done, I find it remarkable." Mackay was first-team coach at Watford when Rodgers was manager in 2008/09. "He's going to go on and become a big manager at a top club and I find it astonishing there's talk about him leaving there."