The Green Party conference passed a motion on Sunday declaring that Zionism is an ideology supporting "an ethnonationalist Jewish state" and calling for a single democratic Palestinian state in all of historic Palestine, including modern-day Israel.
Lubna Speitan, a British-Palestinian artist who proposed the motion, said the aim was to voice opposition to "racist systems of oppression". However, Jewish groups described it as an attack on their community, and some Labour and Liberal Democrat MPs referred the party to Great Britain's equality watchdog.
In response, Israel banned Green Party leader Zack Polanski from entering the country.
On Tuesday, it emerged that court documents from 2015 named Speitan as a "close friend" of Minh Quang Pham, who was jailed for 40 years after pleading guilty to supporting al-Qaeda. Pham was accused of plotting a suicide bomb attack at Heathrow Airport, which he denied. Speitan offered to testify as a defence witness in his trial and wrote a letter to the judge in 2016 describing Pham as "one of the kindest, honest, most sincere" people she had known.
The Green Party said Speitan holds no official positions and said it "takes all allegations of this nature seriously". The BBC has contacted Speitan for comment.