A London-based correspondent of Kremlin-funded news channel Russia Today has resigned in protest at its coverage of the shooting down of Malaysia Airlines flight MH17.

Sara Firth, who worked at Russia Today for five years, described the channel’s reporting of the crash in which 298 people were killed, including a former BBC journalist, as the “straw that broke the camel’s back”.

Russia Today, which has been criticised as a propaganda mouthpiece for the Russian government, suggested Ukraine was to blame for the crash, while most media organisations have said it was shot down by a suspected Russian-made missile.

“It was the most shockingly obvious misinformation and it got to the point where I couldn’t defend it any more,” Firth told the Guardian.

“When this story broke that was the moment I knew I had to go. I walked into the newsroom and there was an eyewitness account making allegations [against Ukraine] and analysis, if you can call it, from our correspondent in the studio.

“It was just appalling, in a situation like that where there are families waiting to be informed and a devastating loss of life.”

She added: “I have always fought against this argument that RT is an evil network but you wake up and think, that’s just wrong.

“It was not an easy decision, I started my career at RT, and I respect many of the team there. In the end it got to the point where I couldn’t defend it and I didn’t believe there was something to defend. A story like this really highlights it.”

Firth said she previously had a “lot of editorial independence” filming reports for the network but said there had previously been examples of senior editorial interference, and said she had been pulled out of Syria after some “very heated discussions” about the channel’s coverage.

Firth said on Twitter: “I resigned from RT today. I have huge respect for many in the team, but I’m for the truth.”

Sara Firth (@Sara__Firth) I resigned from RT today. I have huge respect for many in the team, but I'm for the truth. pic.twitter.com/mZ1g0R7N0D

In an indication of what was to come, she had earlier tweeted: “RT style guide Rule 1: It is ALWAYS * Ukraine’s fault (* add name as applicable).”

Firth said: “There is bias against Russia but you don’t counter wrong by doing even more wrong. They are putting all this money into making it look like the truth and it’s not, it’s just so sad. It’s so close sometimes to be being great.

“I have always said it’s better to have RT than to not have that perspective, but actually with a story like this and they way they misreport it, it’s quite dangerous, I don’t want to be party to it.”

Sara Firth (@Sara__Firth) RT style guide Rule 1: It is ALWAYS *Ukraine's fault (*add name as applicable)

One story on the RT website posted on Thursday – and later toned down – was headlined: “President Putin’s plane might have been the target for Ukrainian missile – sources”.

The RT press office issued the following statement: “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.

“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.”

Former BBC journalist Glenn Thomas was among the people killed when flight MH17 crashed in Ukraine.

A media officer for the World Health Organisation in Geneva, he was among up to 100 passengers on the flight travelling to an Aids conference in Australia.

A WHO spokesman said he would be “remembered for his ready laugh and his passion for public health” and “leaves behind his partner Claudio and his twin sister Tracey, [who] says he died doing what he loved”.

Richard Porter, the controller of BBC World News in English, said his death was “a terrible tragedy and our hearts go out to Glenn’s family”.

“He was a much loved and respected colleague and the BBC World News team are reeling from this very sad news about their friend,” he told BBC News.

“He was always passionate about telling the stories that mattered and could always turn his hand to whatever was asked – but most importantly he was a genuinely nice man.”

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