Ms Firth told Channel 4 News that she believes RT was “disregarding the facts”, and adds that she was not put onto stories about Ukraine because she is a journalist who “asks questions”.
RT has hit back at claims that its reports are not truthful.
“We were not surprised by Sara Firth’s decision to leave RT after five years as a Moscow and London correspondent, as she has recently informed us that she was likely to take an offer from another firm,” RT’s press office said in a statement.
“The plane crash is an absolutely terrible tragedy, there are a lot of questions that surround it and everybody is looking for answers. More than 15 RT journalists are working on this story — from Moscow, from the Russian-Ukranian border, London, Berlin, Washington.
“RT Spanish journalist Francisco Guaita was one of the first TV correspondents to get to the scene late last night. Our reporters are in the Netherlands and Malaysia.
“Sara has declared that she chooses the truth; apparently we have different definitions of the truth. We believe that the truth is what our reporters see on the ground, with their own eyes and not what’s printed in the morning London newspaper.
“In our coverage, RT, unlike the rest of the media, did not draw conclusions before the official investigation has even begun. We show all sides of the story, even if everyone else has already decided which side is to blame.”