Zion Lights, writing in the Daily Mail, also said that she had become unable to defend some of the group’s claims.
XR “peddle messages of doomsday gloom that alienate” and offer “little in the way of positive solutions”, she added.
The group calls on governments to take immediate action on climate change.
It describes itself as an international “non-violent civil disobedience activist movement” and has been involved in a number of high-profile protests since it was formed in 2018.
Last week it targeted UK newspapers – which it has accused of failing to report on climate change – by blocking printing presses and delaying distribution.
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Ms Lights wrote articles for both the Daily Mail and the Daily Telegraph on Thursday explaining her decision to leave behind XR and support nuclear power.
She told the Mail she initially joined XR because its message was “listen to the scientists” and the role of spokesperson gave her a platform “to talk about what I truly felt mattered”.
However, she says she began to rethink her support for the group after an appearance on the BBC’s Andrew Neil Show last October.
Ms Lights said: “It’s a headline-grabbing assertion – but unfortunately, it’s also not true, or certainly not backed up by any evidence. As was obvious to anyone who knows me – and even to the casual viewer – I was plunged into a PR nightmare.
“I could not defend the number, but as the official spokesperson nor could I be seen to condemn it. All I could do, instead, was flounder under the hot glare of the studio lights for what felt like an eternity.”
Ms Lights, who began campaigning about the environment as a student in the early 2000s, said she also had doubts about XR’s approach of telling people “what not to do” and “peddling the notion that the solution to the climate crisis was to turn back the clock to a simpler time”.
Writing in the Telegraph, she said the campaigners who argued that we needed to all live with less – as she once did – had to accept this was not going to happen “and look to solutions instead”.
She said that “many within XR argue in favour of replacing fossil fuels entirely with renewables” but this was not a realistic option and she favoured “a pragmatic approach, rather than peer-group tribal pressure to stick to an outdated mainstream green line”.
Much of the green movement was “steeped in an anti-nuclear mindset”, she said, “when any rational, evidence-based approach shows that a strategy including nuclear energy is the only realistic solution to driving down emissions at the scale and speed required”.
She denied she was making a U-turn, instead saying it was a “logical next step” in looking for solutions rather than “shouting ever more loudly about the problem”.
Ms Lights said she has since taken a role at campaign group Environmental Progress UK, whose campaigns include supporting the building of the Sizewell C nuclear power station in Suffolk.
Nuclear power is planned to be a key part of the UK’s future energy strategy.
BBC environment analyst Roger Harrabin said its key benefit is it helps keep the lights on while producing hardly any of the CO2 emissions that are heating the climate.
However, he said a big environmental problem is what happens to nuclear waste.
A spokesperson for XR said: “In a high stakes scenario you look at the worst-case and plan based on that, because the risks are high enough to warrant taking the risk seriously. It’s an approach employed across the board in all industries. Scientific uncertainty has always been twisted to prevent risk-based analysis.
“Extinction Rebellion has always taken the view that the public deserve to know what credible scientists believe is the worst-case scenario for the development of the climate and ecological crisis over the coming decades.”