Speitan, a British-Palestinian artist, proposed the Green Party conference motion defining Zionism as an ideology supporting “an ethnonationalist Jewish state”.
The motion – which was approved by party members by 1,022 votes to 813 – also called for a “single democratic Palestinian state in all of historic Palestine”.
Under Green Party rules, motions supported by members become official party policy, but will not necessarily be included in the party’s manifesto at the next general election.
The motion attracted criticism from opposition parties and some Jewish groups, who labelled it an “attack on the Jewish community”.
Four out of five Green Party MPs also opposed the motion.
Adrian Ramsay, a former party leader and Green MP for Waveney Valley, said the motion was an “unhelpful distraction” and “wrong in principle” as “the Green Party has always supported the right to self-determination and that has to apply to all peoples”.
Asked if he would consider leaving the party, Ramsay told Matt Chorley on BBC Radio 5 Live he was “not ruling anything in or out at this stage”.
“What I’m doing at this stage, and we’re only a few days after the motion, is to talk carefully with colleagues to reflect both with Jewish Greens but also with Green MPs and other people in leading roles in the party to look at how best can we get things back on track,” he said.
On Tuesday, the Jewish Chronicle reported, external the existence of a 2015 letter from Speitan in which she offered to testify as a defence witness in the trial of Minh Quang Pham.
US prosecutors said Pham had been directed by al-Qaeda leader Anwar al-Awlaki to detonate explosives in Heathrow’s arrivals area after spending several months in Yemen being trained by him.
He was jailed for 40 years in the US after pleading guilty to supporting al-Qaeda. He denied planning a suicide attack.
In US court documents from 2015, Speitan was listed as a “close friend” who could provide “critical testimony regarding Pham’s mental state”.
In a later letter to the US District Judge, Speitan said Pham was one of the “few people I regard with such high respect and esteem”.
She said if the judge “truly knew the Minh [Pham] that I and others grew to love and respect”, they may have “probably had the whole case dismissed”.
She acknowledged he “may have had and developed some behavioural issues” but added: “He turned out to be one of the kindest, honest, most sincere, trustworthy and respectful people I have ever known.”
There is no suggestion in the documents Speitan knew of Pham’s criminal activities at the time they knew each other.
Following the emergence of the letter on Tuesday afternoon, a Green Party spokesperson said: “Where members have expressed views that do not fit in with the party’s values, we have internal processes to investigate.”
On Wednesday, the party released an updated statement adding “an investigation is under way”.
The spokesperson said: “Lubna Speitan holds no party positions other than being a member.”
