Simon Thomas has been Plaid Cymru AM for Mid and West Wales since 2011
Simon Thomas has resigned as a Welsh Assembly Member and quit Plaid Cymru.
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.
In a statement, the Assembly's Presiding Officer Elin Jones said: "The Llywydd has received a letter of resignation from Simon Thomas with immediate effect."
As the announcement of Mr Thomas's resignation came, Plaid Cymru chair Alun Ffred Jones said the party was "aware of a police investigation into allegations of a serious offence".
"Due to that ongoing investigation it would not be appropriate to comment further at this time," he said.
The former librarian, who lives in Aberystwyth, was a special senior advisor to former deputy first minister Ieuan Wyn Jones before he was elected to the assembly.
The resignation of Mr Thomas, a former leadership candidate in 2013 before dropping out, will not trigger a by-election as he was a list member.
In something of a state of shock. I have now got some big decisions to make in the next few days. I won't be on here a lot. Thanks for good wishes already recieved
— Helen Mary Jones (@HelenMaryCymru) July 25, 2018
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 Joneswho ran to become an MP in 1992 and 1997was 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."