Simon Thomas has resigned as a Welsh Assembly Member and quit Plaid Cymru after being arrested by police investigating allegations of a serious offence.
The 54-year-old married father-of-two was elected as Plaid AM for the Mid and West Wales region in 2011.
Mr Thomas, who was MP for Ceredigion between 2000 and 2005, has deleted his Twitter page and personal website.
The Assembly’s Presiding Officer Elin Jones confirmed his resignation.
She announced in a statement that “the Llywydd has received a letter of resignation from Simon Thomas with immediate effect.”
Plaid chairman Alun Ffred Jones said “it would not be appropriate to comment further” due to the ongoing inquiry.
As the announcement of Mr Thomas’s resignation broke, Mr Jones added the party was “aware of a police investigation into allegations of a serious offence”.
The former librarian, who lives in Aberystwyth, was a special senior advisor to former deputy first minister Ieuan Wyn Jones after losing his seat at Westminster and before he was elected to the assembly.
The resignation of Mr Thomas, a former leadership candidate in 2012 before dropping out, will not trigger a by-election as he was a list member.
Former AM Helen Mary Jones, who is next on the Plaid list in the Mid and West Wales region, will now decide whether she wants to replace him.
Ms Jones – who ran to become an MP in 1992 and 1997 – was an AM between 1999 and 2011, firstly representing Llanelli then the Mid and West Wales region.
“In something of a state of shock,” the 58-year-old, who now works at Swansea University, said on Twitter.
“I have now got some big decisions to make in the next few days.”
Simon Thomas quits as Plaid Cymru AM after police arrest}