Alli was forced off in the closing stages of Tottenham's 2-1 win at Fulham on Sunday and scans have confirmed the extent of the star's injury.
The 22-year-old's absence is a massive setback for Tottenham, who are already without England striker Harry Kane until March due to an ankle ligament problem.
Tottenham manager Mauricio Pochettino is also without South Korea forward Son Heung-Min, who is on international duty for several weeks.
“Following scans and clinical assessment, we can confirm that Dele Alli has suffered a hamstring strain,” a statement on Tottenham's website said.
“Dele will now undergo a period of rehabilitation with our medical staff with the expectation of returning to training in early March.”
Alli is set to miss a host of crucial games, including Tottenham's Champions League last 16 tie against Borussia Dortmund.
His injury also puts a huge dent in third-placed Tottenham's slender hopes of chasing down Premier League leaders Liverpool.
Tottenham will now face Chelsea in Thursday's League Cup semifinal second leg without three of their key players.
England midfielder Eric Dier, just back from appendix surgery, is a contender to step in for Alli, while Fernando Llorente deputised for Kane in the win over Fulham
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