Catfish and the Bottlemen guitarist Johnny Bond has quit the band, describing it as "dysfunctional".
In an Instagram post, Bond confirmed he had left in March 2021 but stepped in as a session guitarist for four shows last summer.
The post described how he felt "the professional and personal relationships had become entirely dysfunctional", and certain behaviour became "intolerable".
The Welsh rock band's label, Island Records, has been asked for comment.
The band recently pulled out of their gig at the Principality Stadium "due to unforeseen circumstances".
They had been due to support the Stereophonics alongside Tom Jones.
Bondy, as he is known to fans, said he was receiving a large volume of queries about the band's future, but was not affiliated with them anymore.
He also thanked fans for their support.
He stated: "You have given me some incredible experiences that will stick with me forever and I have enormous love and respect for you all."
The band, formed in Llandudno, Conwy county, were named best British breakthrough act at the 2016 Brit Awards.
They also reached number one in the UK with their second album, The Ride. Their third, The Balance reached number two in the UK.
They are due to perform at the Reading and Leeds Festival in August.