Senior UKIP Wales figure quits party amid candidate row
UKIP Wales' campaign manager for the 2015 general election and 2014 European election is resigning from the party.
John Atkinson said an internal row over assembly candidate selection meant the party was "taking its eye off the ball" on the EU referendum.
He also said he had "no faith" in the people in the running for regional candidate selection.
UKIP Wales leader Nathan Gill said he stood by "John's decision to follow his own moral conscience, as must we all".
Mr Atkinson told BBC Wales: "All the in-fighting over the selections means we're taking our eye off the ball on the referendum.
"I've got no faith the selection system we've finally settled on for the assembly will deliver decent candidates.
"It's a flawed system and it's going to be a lottery."
Mr Atkinson is UKIP Wales treasurer and works as an assistant to Mr Gill.
He stepped down from his general election role early in the 2015 campaign, citing excess workload.
Mr Atkinson was also the UKIP candidate for Carmarthen West and South Pembrokeshire.
Party members are currently voting for regional list candidates for the assembly selection.
The candidates were originally going to be selected by a central committee, prompting concern from some UKIP Wales activists that high-profile figures from England would be imposed on them.
Mr Gill said: "I want to thank John for his years of hard work during such a critical growth period in the party.
"There are lots of different opinions on the party's ongoing process of selecting candidates and I stand by John's decision to follow his own moral conscience, as must we all."