The 20-year-old winger was linked to Tottenham in January.
However, he could not agree on terms with the Premier League club at the time.
Now he is signing for Cardiff, who have moved ahead in the race for his signature.
Spurs still have some interest in Tanner, while Brighton and Swansea were also circling.
Tanner will undergo a medical and sign a three-year contract with Cardiff, per The Sun.
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