John Mills, Labour's biggest individual donor, has warned that a Jeremy Corbyn victory in the party's leadership contest could unleash Conservative "dominance" in British politics.
The businessman, writing exclusively for IBTimes UK, said Labour would "vacate" the political centre if the socialist firebrand secured a shock win, adding that his election would leave David Cameron and the Tories free to implement their "policies untrammelled by an effective opposition".
"This may make left-leaning Labour Party members feel uncompromised by the threat of realpolitik, but it is hard to see that this is going to be good either for the country, or the future of the Labour Party – or for those whom Labour has always tried to represent against the vested interests of the rich and powerful, who have always been inclined to find the Conservatives more amenable to their concerns," Mills said.
The donor, who gave the party £1.65m ($2.47m, €2.32m) in shares of his TV shopping company JML in 2013, was also critical of Labour's general post-election performance.
"Labour seems to have almost nothing to say at the moment about the economy, despite the dismal record chalked up by the Conservatives, only partially relieved by the recent very modest recovery – which is unlikely to last – in the growth rate," Mills argued.
The intervention comes after a shock poll put Corbyn at the front of the Labour leadership race. The survey, conducted by YouGov for The Times, put the 66 year old on 43% among party supporters, with Andy Burnham on 26%, Yvette Cooper on 20% and Liz Kendall on 11% on first preference votes.
Mills concluded: "For those of us who think that Labour exists to try to make the world a better place and that we have to have a Labour government to make this possible, seeing the party in danger of being little more than a pressure group with no prospect of executive action is not where we want to be."
"People who say their heart is with Corbyn, get a transplant", the former prime minister said. Blair also urged the party to resist a lurch to a left and called for the red outfit to "move on" ahead of Labour supporters going to the polls to select Ed Miliband's successor.
"But don't for heaven's sake move back. If we do, then the public won't vote for us – not because our thoughts are too pure, but because our thoughts are out of touch with the world they live in," the 62 year old said.
But Corbyn, who campaigned against Britain's involvement in the 2003 Iraq war, hit back at Blair and claimed his "big problem" was the long-awaited publication of the inquiry into the military intervention.
"Yes, we did win the 1997 election. We lost support consistently after that, and he led us into a disastrous illegal war," he told Sky News.
The Islington North MP, meanwhile, published a summary of his top economic policies, revealing that the Labour leadership hopeful is in favour of a £10 minimum wage.