The rail minister, Lord Peter Hendy, has reported himself to the police after he was spotted using a mobile phone while driving a vintage Routemaster bus in London during a charity tour.

The former Transport for London commissioner apologised for what has been described as an “error of judgment” after a passenger saw him texting behind the wheel of the double-decker last month.

He was reported to the Metropolitan police on 31 March by the passenger who had taken part in the tour three days earlier. It is understood the peer had been contacting a friend about a prostate cancer test during an event to support the Railway Family charity.

A spokesperson for Hendy said: “Last month Peter Hendy used his phone while driving. He has apologised in full for this error of judgment and has contacted the police.”

The Metropolitan police confirmed the case had initially been closed due to lack of evidence but was later reopened after Lord Hendy admitted to using his phone behind the wheel. He is expected to receive six points on his driving licence and a fine.

The organisers of the Railway Family fundraiser said they had not received any formal complaint at the time of the incident.

They said: “Since then, the organisers have only had one conversation from the complainant in the form of a text message, which he sent on 2 April, where he informed us that he’d reported Lord Hendy to the Metropolitan police.

“The Railway Family fundraiser is run by railway professionals, all of whom work with safe systems of work and rules and regulations about safe practice. None of us would advocate or condone unsafe practices. However, we reiterate that we are not aware of the whole situation, and affirm our hope that any investigation is able to be conducted fairly.

“The organisers regret that a positive and enjoyable event ended in this way and would once again like to express our thanks to Lord Hendy for his support for our event and fundraising.”