My flatmate is subletting me the room. Its ilegal right.?

Usually the terms of subletting and property use are determined and defined specifially with the rent contract signed by the original occupants agreed. If said contract specifically forbides subletting u could have a problem, otherwise it is ok; albeit being gauged rent money. Judging by the wording you are in the U.K. ...so also check your local laws specific to your locale, some places specifically forbid such arrangements. My first action would be to view the original contract and terms agreed upon by your flatmates. Just make sure to sign a document or write up an agreement after viewing property arrangements, it is important to see documents and attend to signatures when dealing with agreements or property of any kind.