The Conservative candidate for mayor will be selected in an open election, City A.M. has learned.
London voters who have been registered as members of the Tory party for at least three months will be able to participate in an online ballot, according to one of the party's five candidates.
Non-Tory voters will also be able to participate in the online ballot if they are already registered to vote in London and pre-register for the online primary, with individual candidates allowed to spend up to £100,000 in the nominating process.
Ivan Massow, who declared his intention to run for mayor last month, said he was "delighted" at the news.
"I've spent the last almost year working towards a mayoral bid knowing the only way I could win it is with this primary," he told City A.M.
"This blows the competition wide open. Had it been just Conservative members it would have been very difficult for me to have beaten Zac [Goldsmith], who's very popular in west London, where most Conservative members live.
"My pitch to the party is a broader appeal."
The online ballot will be done under an alternate voting system, in which votes for losing candidates are redistributed.
Voting will take place, and a winner will be announced, in September, while an official shortlist is expected to be finalised later this summer.