During the Scottish Independence campaign, big business came out strongly in favour of the Union - independence would have been detrimental to the interests of business. John Redwood MP has warned companies and big business not to interfere in a potential referendum in the UK on EU membership (Guardian coverage is here). This is akin to asking the Cookie Monster not to eat cookies.
Redwood (who I incidentally saw at Southampton airport last week) has suggested that their customers, employees and shareholders would not like large corporations interfering in the referendum. Redwood bases his argument on the idea that CEOs and corporate tycoons speak on their own behalf and do not represent the interests of their shareholders or other stakeholders. He therefore suggests that stakeholders and shareholders coerce corporate executives into staying out of the debate over the UK's membership of the EU by threatening to destabilise their businesses and corporate governance. Unfortunately, I don't think that this will work - large corporations will want to keep trade barriers low and will want access to cheap European labour. Big business will not want a Brexit.