Construction equipment giant JCB is quitting the CBI business lobbying group amid reports of clashes over Brexit.
Lord Bamford, whose father founded JCB in 1945, has been an outspoken supporter of leaving the EU, while CBI members have warned against it.
Both parties confirmed the decision on Monday night, but declined to offer reasons why.
The move will be seen as a blow for the CBI, which will lose the backing of one of the country's biggest manufacturers.
A JCB spokesman said simply: "I can confirm that JCB is ending its membership of the CBI."
He said he would not be commenting on the reasons for the decision, describing it as "speculation".
A CBI spokesman said: "It's always a shame to see any member leave the CBI, but we recognise that businesses have competing priorities and we respect that."
JCB employs around 5,500 UK workers at 11 factories in Staffordshire, Derbyshire and Wrexham, and operates other plants in North America, India, China and Brazil.
Lord Bamford's support for leaving the EU was underlined when Vote Leave - the official "out" campaign - confirmed this summer that it had received two donations of £100,000 from JCB.
On Sunday, leaders of the CBI expressed alarm at the apparent direction of Government policy towards a hard Brexit option, which they say could see tariffs imposed on 90% of British exports.
The business chiefs have demanded Theresa May rules out the "worst options" of a hard Brexit, such as not having "passporting" arrangements which allow easy access to European markets for the financial services sector.
Business leaders with the CBI and manufacturing body the EEF also signed an open letter in the days after the EU referendum warning of the dangers of a hard Brexit as they urged the Government to "give certainty" on the issue.