Green MP Caroline Lucas has quit as a patron of the Stop the War Coalition after the group issued a series of controversial statements on Syria and terrorism.
Ms Lucas, who had served as vice-president, said she was "stepping back" due to work commitments and because she "didn't support" some recent positions it had adopted.
The decision could increase pressure on Labour leader Jeremy Corbyn to stay away from a Stop the War fundraising event he is due to attend on Friday.
A number of his MPs have voiced disquiet that Mr Corbyn is continuing to associate with the group he chaired before becoming leader, with former frontbencher Tristram Hunt among those branding it "disreputable".
The group put out a tweet and article after the Paris attacks suggesting the city "reaps the whirlwind of western support for extremist violence in Middle East".
Both were subsequently removed and replaced with one saying there "can be absolutely no justification" for the atrocities which left at least 129 people dead.
Andrew Murray, who replaced Mr Corbyn as chairman in September, has also dismissed the idea Labour MPs who backed the bombing of Syria were motivated by "conscience".
Other statements have described Britain as "the most aggressive country in Europe over the last 15 years" and rejected Labour MPs' concerns about aggressive lobbying as "whining complaints".
A spokesman for Ms Lucas said: "Caroline stepped back from the Stop the War Coalition a few weeks ago.
"Her busy parliamentary and constituency schedule means that she doesn't have time to fully engage with the role of a patron and, in light of some recent STWC positions that she didn't support, she felt standing down was the responsible thing to do.
"Like the Stop the War Coalition, Caroline is opposed to British bombing in Syria because it will neither keep Britain safe nor help bring about a lasting peace in Syria."