West Ham will take legal action against a Sporting CP director after he claimed that the Hammers had not made an offer for midfielder William Carvalho.
Communications director Nuno Saraiva said on Saturday that no official bid had been made for the Portuguese international.
"No proposal arrived for the player William Carvalho," he wrote on Facebook. "Mr Sullivan has a duty to prove what he says. For this reason, Sporting Clube de Portugal challenge him to publicly show the proposals he says have been made, as well as the evidence that Sporting CP has received them."
West Ham have responded and insisted that they have written proof that a proposal was made for the player.
David Sullivan Jr, the son of West Ham's co-chairman, took to Twitter to confirm that the club will take legal action against Saraiva.
"West Ham are commencing legal proceedings against the communications director of sporting Lisbon as a written offer for the player was made," he wrote.
"To Bruno de Carvalho the president of sporting Lisbon. To say we never made an offer is nonsense and serious libel."
Sporting were believed to have rejected a club-record bid from West Ham for Carvalho, but then attempted to initiate a deal late on deadline day.
"It is no secret that we made a club record bid for Sporting Lisbon's William Carvalho but unfortunately that offer was rejected a couple of weeks ago," Sullivan said, quoted by Sky Sports.
"Late last night Sporting Lisbon made contact to accept the original offer but unfortunately it was just too late in the day, and we simply did not have enough time to put the player through a medical.
"We were not prepared, as a club, to buy a player for that amount of money without him having gone through adequate medical checks."