Chelsea have handed defender Tomas Kalas a new three-year deal.
The 22-year-old Czech will spend this season on loan at Middlesbrough, after a successful period at the Riverside in the latter half of the last campaign.
Kalas joined the Blues in January 2011 from hometown club Sigma Olomouc and made his first-team debut in a Capital One Cup win over Arsenal in October 2013.
His first start came in April 2014 in the 2-0 victory over Liverpool at Anfield, where he impressed at the heart of defence alongside Branislav Ivanovic.
Kalas was also part of the Czech Under-21 squad for this summer’s European Championships.