US investment bank JP Morgan has described a Brexit as a "terrible deal" for the British economy, warning it could cut up to 4,000 jobs if the UK chooses to leave the European Union later this month.

Speaking alongside Chancellor George Osborne in Bournemouth on Friday (3 June), JP Morgan's chief executive Jamie Dimon indicated the bank "may have no choice but to reorganise our business model here" should the 'Leave' campaign have its way at the 23 June referendum. "Brexit could mean fewer JPMorgan jobs in the UK and more jobs in Europe," he added.

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The lender, the world's sixth-largest bank by total assets, employs 18,000 people across its UK operations in Basingstoke, Edinburgh, Glasgow, Swindon and London, which serves as JP Morgan's European headquarters.

However, Dimon warned the workforce would suffer a drastic cull in the event of a Brexit, with up to 4,000 jobs potentially set to go.

"After a Brexit we cannot do it all here and we will have to start planning for that," he said. "I don't know if it means 1,000 jobs, 2,000 jobs - it could be as many as 4,000, and they will be jobs all around the UK.

"If the EU says anybody who does business as a bank with an EU company has to be based in the EU, you're talking about 3,000-4,000 JP Morgan jobs."

The American lender is the biggest private employer in Dorset and Dimon conceded that, should Britain leave the 28-country bloc, the process of reorganising its operations in the UK could begin as soon as the 24 June.

"At a minimum, a Brexit will result in years of uncertainty and I believe that this uncertainty will hurt the economies of both Britain and the European Union," Dimon said.

"In a bad scenario — and this is not the worst-case scenario — trade retaliation against Britain by countries in the European Union is possible, even though this would not be in their own self-interest."

The Chancellor stressed it was time for the 'Leave' campaign to stop telling voters that jobs would be more secure outside the EU.

"Today, 10 of the largest companies in our services sector [...] are all telling us that there will be damage to our economy and jobs will be at risk if we leave the EU," Osborne said.

"Let's end this deception that somehow if you quit the EU jobs won't be at risk. Our analysis shows in the services sector alone 400,000 jobs could be at risk."

However, Steve Baker MP, a spokesman for the pro-Brexit movement urged the 'Remain' campaign to engage in a honest discussion.

"The British people will not be bullied into voting to hand more money and more power to Brussels by someone whose bonus would make even some eurocrat's eyes water and whose bank helped crash the economy," he said.

"It's time for the 'In' campaign to engage in an honest debate, not make unsubstantiated and illogical threats which are the real danger to our economy."