They are being accused of bullying workers by Brexit-backers
The Confederation of British Industry (CBI), which speaks for 190,000 businesses in the UK, has mobilised dozens of businesses - from Siemens to BT and Airbus - to write to employees explaining why their bosses want to remain part of Europe in a bid to swing the vote in favour of David Cameron's government on June 23rd.
The business lobby group told Sky News that CBI members employing over 800,000 workers have talked to staff about the implications of Brexit - and it expects more employers to engage with staff in the final weeks of the campaign.
Carolyn Fairbairn, director general of the CBI, insisted the initiative was "not about telling people how to vote" but rather a move by "responsible business leaders" to explain to staff "what impact a Brexit would have on company growth, their jobs and their communities."
Revelations of chief executives' involvement in an extensive ground war is likely to further fan tensions between Vote Leave and No 10, which hit new lows on Monday night after the Brexit campaign accused the Prime Minister and Downing Street of "doing deals with businesses ... to help them buy the referendum and save their jobs."
The outspoken attack on Mr Cameron came after the Vote Leave campaign got hold of a leaked letter between the chief executive of Serco, the outsourcing firm which has multibillion-pound contracts with the Government, and the Prime Minister on the matter of mobilising the pro-EU business voice ahead of the referendum.
Rupert Soames, Serco's chief executive, discusses how to "mobilise corporates to look carefully at the risks Brexit represents" and persuade Britain's 500 biggest firms to include Brexit in the risk sections of annual reports due to be published before the referendum date. He then goes on to lobby for his own business interests - expounding the case for privatising more prisons.
Vote Leave seized on the letter, written in February, as evidence that the Prime Minister "has been doing deals with businesses to exaggerate the risk of a Vote Leave" and challenged Mr Cameron to come clean on "how many businesses he has cut secret deals with."
Steve Baker, co-chair of Conservatives for Britain, said the letter was "proof that big corporates are being asked to gang up on hard-working families to try to bully them into staying in the EU."