Tottenham have agreed a €5 million [£3.96m] deal with Sporting for England Under-21 international Eric Dier, Goal understands.
The central defender has accepted terms on a five-year contract at White Hart Lane, which he is due to sign on Thursday, with the Portuguese outfit not retaining a sell-on clause.
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Dier signed professional terms with the Lisbon club in 2010, was loaned to Everton for half a season the following January, and has made 30 first-team appearances for Sporting over the last two campaigns.
Eligible for citizenship in Portugal, Dier has resisted approached from the Portuguese federation to play for their national teams, and has been capped by England at every youth level from Under-18 upwards.
Tottenham have been working on strengthening their options in central defence for several months amidst doubts over Michael Dawson's long-term fitness and with Younes Kaboul's contract entering its final year.
Often played out of his preferred central position as a left back, Jan Vertonghen has been offered a revised five-year-deal and recently stated that he "would like to stay at Tottenham".
New manager Mauricio Pochettino has taken a direct role in approaching potential new recruits, meeting earlier this week to discuss the transfers of Portuguese defenders Jose Fonte and Miguel Lopes.
Fonte, 30, has just a single season left on his Southampton contract and would be allowed to leave the South Coast club for a reasonable transfer fee.
Sporting right-back Lopes spent last season on loan at Lyon and has been identified as a replacement for Kyle Naughton if the former England Under-21 international is sold.