The search for Welsh Labour's next leader should not use the system which saw Jeremy Corbyn elected as UK leader, one of his main Welsh supporters has said.
Mick Antoniw said using the one member, one vote (OMOV) system would be a mistake for "short-term gain".
Instead he feels keeping the current system would help preserve the party's links with trade unions.
It gives equal weight to elected members, unions and ordinary members.
First Minister Carwyn Jones has not announced a date for his departure, but Welsh Labour has been consulting on whether to change the rules.
Welsh Labour is considering whether to adopt OMOV for leadership contests here, which would see a switch from the current electoral college system.
Currently the votes of three group's - elected members (AMs, MPs and MEPs), trade unions and other affiliated groups, and ordinary members - are weighed equally.
In an article on Labour List and writing in a personal capacity, Mr Antoniw said: "The threat of ending the electoral college for electing our leader and deputy would weaken and undermine our historic friendship with trade unions."
"It is easy, and understandable, that there is a temptation for the left within Labour to advocate a simple copy-and-paste of those changes into Welsh Labour's rulebook in the belief it will deliver a similar outcome here," he wrote.
"I totally disagree.
"For potential short-term gain, if we were to end the electoral college and reduce our strong links with the unions by embracing OMOV, then we would undermine Welsh Labour's strength and ability to represent the people and communities of Wales."
Asked if he thought OMOV would produce a left-wing leader in Wales, Mr Antoniw said: "I suspect it would not make much difference."
He said: "Wales has traditionally adopted a different, and more radical position from the rest of England.
"The attempt to stick a very much, England-wide, London-centric constitutional change on Wales would be in my view to weaken the strength of the Labour party in Wales and the Wales Labour party brand."
Ex-Cardiff councillor and secretary of the Corbyn-backing Welsh Labour Grassroots organisation, Darren Williams, said he had not seen Mr Antoniw's article but "would be surprised and disappointed if he's supporting an electoral college rather than OMOV".
Mr Williams, who is also a member of Labour's national and Welsh executive committees, said OMOV was "simply the most democratic system".
"I respect Mick and have complete faith in his sincerity, but some of those who support electoral college seem to do so because they see it as the best way to block a pro-Corbyn candidate," he said.
A Welsh Labour spokeswoman said the Welsh executive committee would consider responses to a consultation on possible party rule changes in Wales when it met next.