The highly-regarded attacker has held talks over a season-long loan deal from Spurs.
And the Blues are confident of completing the switch ahead of this weekend’s League One opener at Sheffield Wednesday.
The 18-year-old Scarlett would inject pace into Pompey’s side, a missing characteristic Danny Cowley has been desperate to add since Marcus Harness’ exit.
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Scarlett is an England under-19 regular and started their 3-1 Euro Championships final triumph over Israel earlier this month.
He has made 10 first-team appearances for Spurs, including two substitute outings in the Premier League.
Pompey’s capture of the attacker would represent something of a coup for Cowley, whose transfer business has quickened considerably during the last fortnight.