West Ham are close to signing William Carvalho, according to Sky sources.
The Portugal international is understood to want a move to the Premier League and has a £40m release clause in his contract, but there is an awareness the deal needs to be done because other clubs have been alerted to his availability.
Despite holding talks over the past two weeks, Sky Sports News understands Sporting will only sanction a deal to sell Carvalho if West Ham match the buy-out fee.
Sky Sports News also understands West Ham have had an opening offer turned down by Sporting for Carvalho, who was left out of the matchday squad for the 0-0 Champions League play-off first-leg draw against Steaua Bucharest.
The 25-year-old, who helped Portugal win Euro 2016, is a product of the Sporting academy and has scored nine goals in 137 appearances since making his first-team debut in 2011.
Sporting have recently signed midfielder Rodrigo Battaglia as a potential replacement for Carvalho.
West Ham manager Slaven Bilic has made five major signings during the summer transfer window, with Pablo Zabaleta, Joe Hart, Marko Arnautovic, Javier Hernandez and Sead Haksabanovic arriving at the London Stadium.
But the Hammers have lost both of their Premier League games so far this season, losing 4-0 at Manchester United and 3-2 at Southampton.