David Cameron's bid to stay in the EU has been given a considerable business boost after US industrial giant General Electric warned that a Brexit would be "hugely damaging".

The company, which employs around 18,000 people in the UK, also hinted that it could move some of its operations out of Britain if the public voted to split from the 28-member bloc.

"I think that an exit from the EU would be hugely damaging and would cause a lot of difficulties in the short term," said Mark Elborne, the UK head of General Electric, as reported by the Daily Telegraph.

Elborne also explained that his company was supportive of the prime minister's plan to gain concessions from Brussels.

"We want to be part of a trading group where we can continue to benefit from the lack of barriers, we want to benefit from large-scale trade agreements, the convergence of standards, and we want to be able to operate seamlessly between our different businesses across Europe."

The comments come after Cameron conceded that the possibility of securing a treaty change from the EU would be unlikely before his promised referendum by the end of 2017.

He instead wants Brussels to agree to make "irreversible" and "legally binding" pledges to change EU law in the future.

"It is the agreement of treaty change – that must happen. But it was never going to be the case that you would get all 27 other parliaments to pass treaty change before you have your referendum," the prime minister said.

Cameron faces increasing pressure from his own party over the issue after a group led by Eurosceptics, Conservatives for Britain, was set-up.

The organisation, which claims it will gain the support of more than 100 Tory MPs, maintains that it is supportive of Cameron.

Meanwhile, Labour has established its own pro-EU campaign group, which will be headed by former Home Secretary Alan Johnson.

"This is not a return to the shadow cabinet, but Harriet [Harman] is always very persuasive. There will be a lot of different kinds of campaigns for a yes vote, but I don't think there has ever been an argument that Labour should not have its own campaign," the former trade union chief told The Guardian.

"As a political party we have to be very clear where we stand on this issue. I see this as the most important political decision of my lifetime."

But Johnson will have to make the case for staying within the EU while debating some members of his own party.

Eurosceptic Labour MPs Kate Hoey and Graham Stringer have helped create Labour for Britain. The organisation wants to push for a "full dialogue" ahead of the EU referendum and has the backing of top Labour donor John Mills.