Richard Corbett, who advises the European Council President Herman Van Rompuy, has said that Brussels sees tonight's vote as an internal debate, adding that "some would say [it is] an internal debate within one particular party".
Speaking to BBC News, he likened Britain leaving the EU to "economic suicide" and said that 3.5 million jobs would directly suffer.
Cameron has pledged to back a Conservative MP's EU referendum bill, as Cable labelled the move potentially 'damaging' for the country.
114 Conservative backbenchers have voted in favour of a eurosceptic motion to amend the Queen's Speech in defiance of the Prime Minister.
David Cameron has denied he has buckled under pressure from his backbench MPs over the EU debate.