Jeremy Corbyn has attacked Sir Keir Starmer for wanting to "prop up" wealth and power instead of challenging them.
Mr Corbyn appeared to suggest the Labour leader only champions ideas that do not coincide with what "the majority of people want".
Writing in the i newspaper, the former Labour leader said: "So far this week, Labour’s leaders have shown they want to prop up, not challenge that wealth and power.
"There is another way forward, that is based on social justice, and in the policies the majority of people actually want, not what the establishment and its media mouthpieces insist they should want."
With COP26 fast approaching Mr Corbyn said "we cannot afford to let the Government off the hook".
"The stakes are too high. In just a few weeks, the world’s leaders will come together in Glasgow to try to thrash out an agreement on climate change.
"Movements from around the world will be putting maximum pressure on our rulers to take the radical action now needed to decarbonise by 2030."
Mr Corbyn's attack comes after Andy McDonald quit his Shadow Cabinet position, accusing Sir Keir of dividing the party and abandoning his pledges.
Andy McDonald said the Labour leader had not honoured “our commitment to socialist policies” as he quit as shadow employment rights secretary on Monday.
The MP, who also served under Sir Keir’s predecessor Jeremy Corbyn, said the leader’s office had instructed him to argue against a national minimum wage of £15 per hour and against statutory sick pay at the living wage.
“This is something I could not do,” Mr McDonald wrote.
