Louis McKechnie, 21, was released from HMP Thameside South East London, on New Year’s Eve after serving half a three-month sentence for breaking an injunction by gluing himself to the motorway.
He was “terrified” before going to prison but found life behind bars “quite manageable”, he told radio station LBC.
Other prisoners said they “supported what we were doing and that they’d make sure our time in prison was OK”, Mr McKechnie recalled.
“My experience with prison has emboldened me to take any future action regardless of if prison is a consequence.”
Asked if he would block the M25 again if Insulate Britain conducts another protest there, he replied: “Absolutely. I feel that if we’re able to save these 8,000 to 30,000 lives every year that are lost to fuel poverty, I would spend the rest of my life in prison for that.”
Insulate Britain, an offshoot of Extinction Rebellion, wants the Government to insulate all UK homes by 2030 to cut carbon emissions.
It repeatedly blocked major roads between September and November last year, causing long traffic jams.
Government-owned National Highways responded by obtaining High Court injunctions which banned protests on motorways and major A roads in England.
Mr McKechnie was among nine Insulate Britain supporters jailed for taking part in a blockade of the M25 on October 8.
He said the other prisoners he met were “lovely”.
He added: “They were actually quite empathetic, especially because a lot of them are from working class backgrounds and a lot of them have experienced fuel poverty themselves, and so they’re absolutely understanding that we need to take action to solve this and the Government isn’t going to act unless pushed to it.”
A recent investigation by the PA news agency found policing the stunts cost taxpayers at least £4.3 million.