Cardiff City owner has reportedly told manager Malky Mackay: "Resign or be sacked", reports BBC Sport.
This declaration comes after a long running feud between Mackay and Tan, most recently over Mackay's desire to sign "three more quality additions".
Mackay declared after Cardiff's 1-0 win over West Brom; "I would look to strengthen all three departments in terms of a defender, a midfielder and an attacking player."
This deeply upset Tan and put Mackay's job security under pressure.
In a letter sent by e-mail to Mackay by Tan the Malaysian pulled no punches as he listed all of his issues with the manager in great detail.
The grievances included results, style of play and, naturally, transfer budgets and signings. He allegedly questioned Mackay's managerial record.
Mackay had previously highlighted his loyalty to Cardiff City, stating he would not resign despite Tan's outrage.
It has been reported that the correspondence between the two occurred on the same day that Tan went public criticising the Scot through chief executive Simon Lim.
Mackay has enjoyed notable success at Cardiff City. Not only did he oversee City's promotion to the Premier League last year, City have managed four wins in the Premier League this season - most notably against Manchester City - as well as a dramatic late draw against Manchester United and impressive pre-season wins against Chievo and Athletic Bilbao.
Mackay has achieved all this under constant pressure and scrutiny from the Malaysian billionaire. Cardiff City sit fifteenth in the table, four points above the relegation zone, with fans often voicing their support for Mackay.
During Mackay's now notorious interview he went on to praise the board's support in the summer, and pointed out that he was awaiting their guidance.
"I will talk to my chairman [about January] who will then discuss it with the board of directors and our owner and I will take my lead from them," he said.
A Cardiff City spokesman said; "The club is not party to any letter sent from one person to another."
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