The influential Tory donors behind the JCB digger empire could be hit with a bill for more than £500m to settle a longrunning investigation by HM Revenue and Customs, the Guardian can reveal. The investigation into Anthony Bamford, a Tory peer, and his brother Mark, the director of a subsidiary of the Conservative party, is understood to span a complex network of offshore tax havens and companies. The inquiry is understood to be targeting efforts by the Bamford dynasty to aggressively minimise the payment of UK taxes and covers two decades. The civil investigation, ongoing for the past three years, is understood to be serious, and of a kind only launched when the HMRC has grounds to suspect a significant loss of tax. It involves questions over the tax due on shares, held offshore in Bermudan family trusts that control the vast JCB empire. Crucially, it centres on when the brothers took ownership of those shares from their father, Joseph Bamford, who died in 2001. The serious nature of the investigation has come to light as the Conservative and Labour parties urgently court potential donors before next year’s general election. JCB, a long-term contributor to the Tories, has also been targeting the Labour party, including handing out free merchandise to delegates at the party’s recent conference in Liverpool in an attempt to win support for the company’s hydrogen ventures. The Guardian revealed last month that the HMRC investigation was under way, but sources claim Lord Bamford has still not informed the parliamentary authorities that he is subject to a serious tax investigation. Sources familiar with the code of conduct for peers believe a leading businessperson would be obliged to inform the house authorities of this kind of HMRC investigation. He has also not offered an account to the Conservative party or the prime minister, Rishi Sunak, according to insiders at its HQ. The party has refused to say whether it continues to accept donations from the family or its businesses and declined to comment on the Bamford investigation. The Bamfords have been among the most influential donors to the Tory party in recent years, with close relationships with several former prime ministers, including Liz Truss, David Cameron – who made Bamford a peer – and Boris Johnson. The family’s gifts and donations total more than £10m over the past 20 years. Johnson has close links with Bamford in particular, with the peer funding his wedding to his latest wife, Carrie, including South African-themed food and a marquee on the lawn of his Daylesford estate in the Cotswolds. The Bamford brothers inherited the JCB empire from their father and have grown it into one of the world’s biggest makers of heavy construction gear, with 11,000 staff, pretax profits of £558m last year and factories from Staffordshire to New Delhi in India. The investigation brings into focus previous public statements by Bamford that suggested his tax affairs were simple – claims that were used to justify a second, successful attempt by Cameron to award him a peerage. Labour has formally written to the chair of the Tory party, Greg Hands, asking him to clarify whether the party was aware of the inquiry and if it would still accept donations while the investigation was ongoing. Lawyers acting for Lord Bamford and Mark Bamford declined to comment on the record. An HMRC spokesperson said: “We neither confirm nor deny investigations and cannot comment on identifiable taxpayers or businesses due to strict confidentiality laws.” The Bamford brothers have been at the heart of the Tory party’s elite and its finances for years. They have also formed close public and private relationships with frontline politicians and sought to steer the course of tax policy and wider public policy through direct lobbying and the rightwing thinktank the Centre for Policy Studies. Sources also fear that the Bamfords’ influence over major figures in the Conservative party could place pressure on HMRC to drop or minimise the outcome of its investigation. It is not the first HMRC inquiry into Lord Bamford’s finances, which are closely intertwined with JCB. The Guardian understands that serious corporate and personal tax queries have led to deals worth millions of pounds with HMRC before this latest investigation. Bamford told the Evening Standard in 2012 that he had “no tax schemes” and that his tax return was a “simple one”. He made the comments to the newspaper after his withdrawal from the first attempt to make him a peer, after it was revealed that officials had shared concerns over his tax affairs with the Lords’ appointments process in 2011. He was eventually made a member of the House of Lords in 2013. The vast bulk of the Bamfords’ wealth and income is derived from their shares in JCB and the dividends paid on them. These shares are held offshore in Bermuda in family trusts at the top of a sprawling offshore network, which the Guardian understands was set up with the aim of avoiding paying UK tax on the dividends and inheritance tax. The years-long HMRC inquiry is examining who was the owner of the shares at the precise time the Bermudan trusts were created to house them in 1996, according to sources. This means officials have been considering the tax due over the past 20 years. If the brothers had owned the shares before 1996, they might not have paid the correct tax on dividends that flowed from those shares, leaving them with a potential bill of more than £500m. That would be composed of the original tax owed, interest on that tax and a penalty of up to 200%, a Guardian analysis of dividend payments and online HMRC guidance suggests.