One of the Conservative party’s biggest and most influential donors, Anthony Bamford, is under investigation over his tax affairs, the Guardian can disclose. The broad-ranging inquiry by HMRC calls into question whether the Tories should accept funds from the peer in the run-up to the general election. It may also cast a shadow over the more than £10m he and his family have given to the party over the past 20 years. The Guardian understands that the HMRC inquiry has been ongoing for three years and covers a period spanning several years. Lord Bamford, who runs one of the UK’s best-known manufacturers, JCB, has been among the most significant donors to the Conservative party in recent decades. He also bankrolled the former prime minister Boris Johnson and the Vote Leave campaign to quit the EU. The Guardian’s revelation that HMRC is investigating the peer over his personal tax affairs risks embarrassment for the Tory party and the House of Lords. Peers are obliged to inform the second chamber’s authorities of anything that has the potential to bring parliament into disrepute and sources said Bamford had not done so. Bamford’s brother, Mark Bamford, who is a director of the Conservative Party Foundation and a major Tory donor in his own right, is also under investigation, the Guardian understands. The Guardian approached Lord Bamford and his brother Mark. Neither had provided comment at the time of publication. HMRC said it could neither confirm nor deny the investigation, and could not comment on identifiable taxpayers, citing confidentiality obligations. The Bamford family have made about £10m in gifts and donations to the party in the past 20 years. Lord Bamford also personally paid for Johnson’s 2021 wedding party and offered the use of his own London townhouse and a cottage to Johnson’s family at below market rent last year. JC Bamford Excavators, the yellow digger company founded by his father, Joseph Cyril Bamford, was the fourth most important source of political party donations for any party in the 2019 election and the Tories’ top donor that year, according to a 2022 study by the University of Warwick. Anthony and Mark are directors of the company. This placed the family in a “super-donor” category, academics found. Another of Lord Bamford’s companies, JCB Research, was also a “super donor” in 2010. Some senior officials and ministers are aware of the HMRC inquiry and have sought information about its progress. The race to build war chests for the general election campaign is under way, with Tory donors being tapped by Conservative campaign headquarters (CCHQ) in order to “keep Keir Starmer out of Number 10”. A general election is due before the end of January 2025. Questions over his tax affairs led to Bamford withdrawing from the peerage appointment process in 2011. He said in an interview with the Evening Standard in 2012: “I knew it was all utterly untrue but thought the best thing to do at that time was to withdraw. I am a UK taxpayer. I am not a non-dom. I pay a very large amount of tax every year personally because I earn a lot of money. “I have no tax schemes. My tax return is a very simple one. I have no outstanding tax matters at all. Have I ever had disputes on PAYE in the last 20 years? Once or twice but not major disputes. But let me emphasise, there are no outstanding tax matters.” Knighted in 1990, Bamford was elevated to the House of Lords in 2013 by the then prime minister, David Cameron. The Conservative party has faced close scrutiny of the tax affairs of its donors and ministers in recent years. Nadhim Zahawi was investigated over his tax affairs while chancellor and the Guardian revealed that he was forced to pay a multimillion-pound penalty to HMRC. The Tory donor and Indian rice tycoon Karan Chanana faced scrutiny by the Indian finance ministry over his tax affairs. For business leaders, this includes bodies such as HMRC, sources said. This is meant to happen at the start of an investigation or as soon as a peer is made aware of it. The Guardian understands that Bamford was informed of the investigation more than a year ago. Bamford has not informed the House of Lords authorities that he is under investigation, sources told the Guardian. He has publicly criticised UK tax policy in recent weeks. In July, he told the Sunday Times that a recent rise in corporation tax from 19% to 25% was a “mammoth increase” and indicative of there being “no appetite for a long-term business policy”. Bamford’s JCB Excavators helped fund Liz Truss’s Conservative party leadership campaign even while he was under investigation over his tax affairs. Boris and Carrie Johnson’s wedding party was hosted and paid for by Bamford at his Cotswolds estate. It was said to have a festival vibe, with the Bamfords footing the bill for South African party food and rum. In 2019 Johnson drove a JCB digger through a wall of polystyrene bricks emblazoned with the words “Get Brexit done”. Johnson, Truss and Zahawi have all received financial support from Bamford or the wider family. Zahawi has received donations from Bamford’s son Jo to help fund the activities of his constituency office in Stratford-upon-Avon, with the latest being a donation of £7,500 in February this year. Mark Bamford gave nearly £1m to the Tory party in March 2022. JCB was founded in 1945 and has 22 factories globally, employing more than 18,000 people, who make more than 300 different products, according to its website. It reported a turnover of £4.4bn in 2022 and made a profit before tax of £501.6m. JCB Excavators is held by JCB Group Holdings based in Switzerland, which is ultimately controlled by family interests.