Chelsea have handed defender Tomas Kalas a new three-year contract.
The centre-back, who can also operate on the right-hand side of defence, last week re-joined Middlesbrough on a season-long loan following a spell with the Championship club last term.
Since joining from Sigma Olomouc in 2010, Kalas has made only two Premier League appearances for Chelsea - against Liverpool and Cardiff City during the 2013-14 campaign.
The 22-year-old has one senior international cap to his name, having featured in the Czech Republic's 3-0 friendly win over Slovakia in November 2012.
Kalas, who has previously spent time on loan at Vitesse and Cologne, will be hoping to catch the eye of Jose Mourinho as the Teessiders attempt to make another push for promotion after losing the play-off final to Norwich City in May.
