Romania’s prime minister, Victor Ponta, said he would stand down as leader of the ruling Social Democratic party on Sunday until prosecutors have completed a criminal investigation against him.

He says the allegations of forgery, money laundering and tax evasion, dating from before he became prime minister, are politically motivated and has denied any wrongdoing.

“There is a new and special situation that I must react to,” he said in a letter posted on his Facebook page and addressed to his leftist Social Democrat party’s senior members.

“The PSD president is being investigated by the DNA (anti-corruption prosecutors) ... in a bid not to let this situation harm the party, I inform you about my decision to hold no positions in the party until I prove my innocence.”

Ponta returned to Bucharest on Wednesday after nearly a month in Turkey for knee surgery, a move that raised questions about his political future and the stability of the government.

He checked into a hospital in Istanbul without informing the president or the Romanian public. His absence coincided with the criminal investigation into his past activities as a lawyer.

As well as dealing with the prosecutors, Ponta is also trying to revive an aid deal with the International Monetary Fund and the European Commission.

The deal was derailed by sweeping tax cuts championed by the 42-year-old Ponta due to come into force in 2016. International lenders say the government must take measures to offset the tax cuts to help safeguard financial stability.