Valencia's Juan Mata was undergoing a medical in west London last night after Chelsea agreed a fee of £23.5m.
The 23-year-old winger announced on Twitter that he was on his way to Stamford Bridge for a medical. Mata tweeted: "Travel to London for medical examination, and come back to Valencia to say goodbye to everybody as you deserve... thanks so much."
The Stamford Bridge club said the deal was subject to Mata agreeing personal terms and passing a medical. A statement on the club's website read: "An agreement has been reached with Valencia for the transfer of midfielder Juan Manuel Mata."
Mata has been at Valencia since 2007, when he joined from Real Madrid's youth team. Last season, the 23-year-old midfielder, who is 5ft 7in tall, scored eight goals to help Valencia to a third-place finish in La Liga. As captain he helped Spain win the Under-21 European Championship in the summer, and he was also part of Spain's 2010 World Cup-winning squad.
When Mata leaves, Valencia will have sold all four of their World Cup winners, with Mata following David Villa (who joined Barcelona), David Silva (Manchester City) and Raul Albiol (Real Madrid) out of the club.
The Chelsea manager Andre Villas-Boas has previously spoken of the need to recruit more midfielders and the club remain keen to sign Tottenham Hotspur's Croatia international Luka Modric.