Labour London assembly members are likely to face full mandatory reselection, which could lead to several prominent party moderates being deselected, a leaked email has revealed.

It is understood that the Labour assembly group leader, Len Duvall, has expressed serious concerns about the proposals, which would overhaul the candidate selection ahead of the next mayoral race

The Guardian has seen an email in which London-wide assembly members (AMs) were told they could potentially face full, open selection by party members. The decision has not yet been agreed by Labour’s governing national executive committee.

There are 14 constituencies across London that each elect one AM by the first-past-the-post system. The remaining nine, of which Labour hold three, are elected from a closed list with proportional representation – meaning that the higher up the party’s list a candidate is placed, the better the chance of being elected.

The email sent last week from the party’s acting London regional director, Andy Smith, said the organisation committee had decided those London-wide members would face “an open selection with members voting to rank where on the list nominees would appear”.

Sources told the Guardian the change was likely to cause concern for the London mayor, Sadiq Khan. The three London-wide Labour AMs are all party moderates.

They are Sadiq Khan’s deputy mayor for fire, Fiona Twycross; the Labour group’s lead housing spokesman, Tom Copley; and Nicky Gavron, a former deputy mayor under Ken Livingstone.

As incumbent AMs, the trio will automatically be on the ballot “but would not be automatically ranked in the highest position on the final list”, the email says.

The new system would likely mean the three lose their positions at the top of the list, should any influential leftwing group such as Momentum decide not to back them. Any new nominees will be based on nominations from constituency Labour parties and regional affiliates, Smith’s email says.

List members have historically been selected by the regional board – a system that has caused past concern for being undemocratic.

However, Duvall is understood to have told the party he is extremely worried that experienced AMs would be given the boot as part of a factional divide.

Smith’s email also says that the constituency AMs, who make up the bulk of the party group, will be subject to a trigger process, broadly in line with that faced by MPs.”

Party members at Labour conference in September voted to back new rules that MPs must face full re-selection if requested by more than one-third of local branches or more than one-third of the local affiliates, such as trade unions.

Momentum has argued that all MPs should face an open selection process at each election, saying Labour’s present system stifles alternative candidates. The group, which controls all of the nine seats for party members on the NEC, is understood not to have been directly involved in pushing for the change, but is likely to be supportive.

“The current system is a straight stitch-up where Labour members have no say in who runs as a Labour candidate. Open selections [are] obviously the most fair and democratic way to choose candidates, and the current system is completely out of step with the mood of the membership.”

The NEC must sign-off the new rules before they come into effect. A Labour source said serving AMs still stood a good chance of keeping their seats even if the changes were accepted.

“It’s possible that you end up with very different outcomes to last time, but it’s only really one small change in the process,” the source said. “Around 92% of 2017 general election candidates that re-stood in early selections were reselected. AM selections will be much the same.”