The 18-year-old is set to undergo a medical in the UK over the next 24 hours and could then fly straight to Asia to join the Spurs squad on their pre-season tour.
Phillips broke into the Blackburn first-team last season, making eight Championship appearances and playing 14 times in all competitions.
He is highly-rated and has represented England at youth-team level.
Speaking about Phillips during his time in charge at Ewood Park, former Blackburn manager Tony Mowbray said: “He's 6ft 4in, he's fast, mobile and can pass it really well. He can use both feet, he's composed.
“Ash is just a phenomenal young boy. If you were to create a defender in the mold of how you would want one, this kid has got every attribute.”
Tottenham’s move for Phillips will not impact their bid to sign a senior centre-back this summer.
Spurs are interested in Bayer Leverkusen’s Edmond Tapsoba and Wolfsburg’s Micky van de Ven as they bid to upgrade their backline.
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