A former Conservative minister has quit the party and thrown her support behind Sir Keir Starmer's "competent political leadership".
Claire Perry O'Neill, who was part of Theresa May's cabinet and an MP between 2010-2019, said the Tories are too "beholden by ideology and self-obsession" to deliver the change the country needs.
"I spend most of my time now working in the private sector and this is not the way to build back confidence and deliver investment - especially in the crucial energy sector," she told The Times.
In an article about how to ensure the growth of renewable energy, Ms Perry O'Neill added: "My former party's often cavalier approach to business and academia coupled with a post-Brexit reluctance to strategically engage with our European neighbours has damaged our ability to deliver the energy system we need."
While she is no longer a member of any party, she lavished praise on Labour leader Sir Keir Starmer and his "sober, fact-driven, competent political leadership".
She said Labour had put "energy at the top of their proposed new government inbox", and applauded its ambitions for green technologies and a national wealth fund to "build British industry".
She said: "Building a low-carbon, secure, affordable energy system for the UK is an immense challenge that needs sober, fact-driven, competent political leadership.
"I sense that is exactly what we will get should Labour win the next general election."
Ms Perry O'Neill stood down as an MP at the 2019 general election but had been expected to lead Britain's preparations for the COP26 climate summit in Glasgow, which was initially planned for November 2020.
Despite being tasked with heading the UK's efforts by Boris Johnson on his arrival in Numer 10, she was sacked in February 2020 and replaced by Alok Sharma.
The endorsement comes after Gareth Quarry, a multimillionaire former donor to the Conservative Party, told Sky News he stopped giving them money and defected to Labour because he felt they were "riven with arrogance and complacency".
In January last year, former Conservative MP Christian Wakeford also quit the party and joined Labour, claiming he was threatened with the loss of a school in his constituency if he did not vote the way party bosses wanted him to.
Ms Perry O'Neill's comments come as Labour continues to enjoy a 20-point lead over the Tories in the polls, with a general election due before January 2025.
Sir Keir addressed Labour MPs in a private meeting to discuss the party's 2023 strategy on Monday night, saying they must use the next year to lay the groundwork for an election victory in 2024 and "redouble their efforts" to win power.
He told the meeting: "This year is going to be all about setting out that plan. So that when people ask us, 'What is Labour for?', our answer is simple: 'To give Britain its future back'.
"We know the work will be tough, no one here doubts that. But we also know the prize at the end is massive.
"The chance to add '24 to '45 and '97 in the history books. The chance to rebuild our country. The chance of the greener, fairer Britain our people deserve."