This article is more than 2 years old

This article is more than 2 years old

A Welsh politician has resigned from the national assembly and left his party as police investigate allegations of a “serious offence”.

Simon Thomas, who was a Plaid Cymru MP before taking up a seat in the Welsh parliament, resigned with immediate effect.

The party’s chairman, Alun Ffred Jones, said: “Plaid Cymru has received Simon Thomas’s resignation as a member of the party.

“We are 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.”

Thomas, 54, was elected as a Plaid assembly member for the Mid and West Wales region in 2011. He was MP for Ceredigion between 2000 and 2005 and stood for the leadership of Plaid in 2011.

The assembly’s presiding officer, Elin Jones, said she had received a letter of resignation from Thomas.

Thomas’s resignation will not trigger a byelection as he was a list member. The former Plaid assembly member Helen Mary Jones, who is next on the party’s list in Mid and West Wales, is in line to replace him.

Helen Mary Jones (@HelenMaryCymru) 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

Asked about Thomas’s resignation, a spokesperson for Dyfed-Powys police said: “An individual from Aberystwyth has been arrested on suspicion of possessing indecent images. The individual has been released on police bail for 28 days.”