MP Jared O'Mara has been arrested on suspicion of fraud, according to multiple reports.
The independent MP for Sheffield Hallam was arrested last Friday and was taken into custody at the same time as his chief of staff Gareth Arnold, the BBC reports.
Police officers were said to have raided the MP's constituency office, removing documents and computers as part of their investigation.
They were both released on Saturday pending further investigation, according to The Mirror.
The newspaper also reports that Arnold had contacted police last month to express concerns about expenses claims and that O'Mara has submitted paperwork for his resignation as an MP.
Last month, the MP announced he would be taking time out from his duties "to receive professional help to deal with my mental health and personal issues regarding self-medication".
The announcement came days after Arnold quit his position by announcing his resignation on O'Mara's Twitter account, although he later claimed to still be working for the MP.
Asked for comment, a South Yorkshire Police spokeswoman told the Standard they could neither confirm or deny the identity of someone who has been arrested.
O'Mara has been approached for comment.
