Veteran Labour MP Nick Brown has had the whip suspended after a complaint was lodged against him under the party’s new independent complaints process. The Guardian understands that the senior politician, the MP for Newcastle upon Tyne East, who has been chief whip for every Labour leader from Tony Blair onwards, is under investigation after a complaint was made against him. Brown, 72, one of Labour’s longest-serving MPs, has also had his party membership suspended until the case is resolved. The nature of the complaint is unknown. The former minister said he had not been told what the complaint was about but that he was “cooperating fully” with the investigation. He is believed to be the highest-profile figure to face investigation under Labour’s new independent complaints process, which was brought in earlier this year after the party was censured by the equalities watchdog over its handling of antisemitism. In a statement, Brown said: “There has been a complaint made about me to the Labour party, which is under investigation. I am therefore under an administrative suspension from the Labour party until the investigation is concluded. I’m not aware of what the complaint is. I am cooperating fully with the investigation.” Sources said Labour’s chief whip, Alan Campbell, informed the party’s parliamentary committee, which represents MPs in their dealings with the leadership, of Brown’s suspension. Under party rules, once an MP has been administratively suspended and is under investigation, they have the party whip suspended as an automatic precaution. Labour’s new complaints process, signed off at the party conference last autumn, covers all protected characteristics, including race, disability and sexuality, and all forms of discrimination. Brown was reappointed as Labour chief whip after Keir Starmer’s victory in the 2020 leadership election, making him the only MP to have held the role for three non-consecutive terms, under six different leaders across four decades. Brown left the role of chief whip for the third time as a result of Starmer’s shadow cabinet reshuffle in May 2021.