Plans to throw open the selection of Labour Party candidates could transform British politics, a former minister has claimed.
A step short of allowing every Londoner a say, it means that 'registered Labour supporters' as well as party members, would be allowed a say.
The so-called 'closed primary' measure was part of Miliband's attempts to fight back after the Falkirk selection scandal, which saw accusations that the Unite union was manipulating the process to instill its chosen candidate.
Lord Adonis, a former Transport Secretary and a key adviser to Ed Miliband, told HuffPost UK he expected the policy to be expanded across the country.
He said: "I think it's a path-making decision in our national politics. This will be the first time that Labour has used primaries, and the decision to use one for the London mayoral election is transformational.
"It will open the Labour selection process to millions of Londoners, and it will have a big effect on how we approach selection for parliamentary candidates.
"This is not going to stop in London."
Adonis admitted the cost of holding primaries was an issue that would have to be addressed in the future.
The Conservatives became the first British party to experiment with primaries in the Totnes seat won by Dr Sarah Wollaston in 2010.
Another Conservative MP, Douglas Carswell, praised the decision, saying a "tiny clique" of people were entering politics under the current setup.
"The pity is that it was not David Cameron making today’s announcement," he said.