Simon Thomas has quit as a Welsh assembly member after being arrested by police investigating allegations of possession of indecent images.
The Plaid Cymru Mid and West Wales AM, a 54-year-old married father-of-two, has also quit the party.
Mr Thomas, who was MP for Ceredigion between 2000 and 2005, has deleted his Twitter page and personal website.
Dyfed-Powys Police said an “individual from Aberystwyth” had been released on bail for 28 days.
A statement from the force said an arrest had been made “on suspicion of possessing indecent images.”
The assembly’s Presiding Officer Elin Jones confirmed Mr Thomas’s resignation in a statement at about 10:00 BST on Wednesday, saying “the Llywydd has received a letter of resignation from Simon Thomas with immediate effect”.
Plaid chairman Alun Ffred Jones said the party was “aware of a police investigation into allegations of a serious offence”.
He added: “Due to that ongoing investigation it would not be appropriate to comment further at this time.”
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 was 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.”