Chelsea defender Tomas Kalas has signed a new contract with the Premier League champions which ties him to the club until the summer of 2018.
Kalas, 22, joined Championship club Middlesbrough on loan for the season last week, but Chelsea have moved to tie down his long-term future.
He has previously spent time on loan at Vitesse Arnhem and Cologne, and returns to Boro after making 17 appearances for them in another loan spell earlier this year.
The Czech Republic under-21 international made his full Chelsea debut in April 2014 in a 2-0 victory over Liverpool at Anfield.
It follows a similar deal involving striker Patrick Bamford, who signed a contract extension before joining Crystal Palace on loan for the campaign.