The 23-year-old spent the whole of last season on-loan at Middlesbrough and now returns to the Championship with the Cottagers, according to the Chelsea website.

The Czech Republic international arrived at Stamford Bridge in 2011 and went on to make his Blues debut during the 2013-14 season, making two Premier League appearances and one Champions League appearance.

Kalas featured 30 times for Boro and helped them to a fourth-place finish in the Championship last season.

Meanwhile, midfielder John Swift has joined Reading on a permanent basis, however, the fee will be decided by an independent tribunal.