A British exit from the EU would isolate the country’s scientists and reduce its influence in medicine, global pharmaceuticals executives have warned, as Prime Minister David Cameron fights for a deal in time for a June referendum.

The warning by executives at some of the world’s biggest drugmakers follows a plea by the UK prime minister for business to enter the debate and comes as the pro-Europe campaign steps up its rhetoric over the potential risks of Brexit, FT reported.

Stuart Rose, head of the Britain Stronger in Europe, set the tone of the wider business campaign on Monday by repeatedly warning of the economic 'risk' if Britain were to leave the EU.

He accepted that both sides of the debate would trade statistics about the benefits or drawbacks of Britain’s EU membership, but told the BBC: “Do we want to sacrifice the imperfect reality of today for the risk of tomorrow?”

Pro-EU campaigners acknowledge the public will quickly tire of the statistical battle but believe that uncertainty — dubbed 'Project Fear' by those in the Leave campaign — will be a potent weapon in the referendum.

Similar tactics were used by the No campaign in the Scottish referendum and Lord Rose challenged his opponents to explain precisely what trading relationship Britain should have with the rest of Europe, were it to leave.

Brexit would be 'a leap in the dark' and Eurosceptics’ argument that Britain could negotiate a free trade deal equivalent to the arrangement it enjoys with other European countries is 'a fantasy', Lord Rose said when he unveiled his research.

Cameron is hoping to hold the EU referendum in June after delivering an agreement with other EU countries about changing the terms of Britain’s continued membership at a summit next month.

But as unrest among Eurosceptics in his Conservative party builds over his tactics, his preferred timescale is under pressure. Speaking in Davos last week he said he was 'not in a hurry' to get a deal done at the February summit if the terms were not right.

Nicola Sturgeon, Scotland’s first minister, on Sunday warned Cameron of the risks of negative campaigning and called on him to delay the vote to give the pro-European campaign time to build a positive message.

She urged Cameron not to hold the vote in June, saying it meant that the campaign would overlap with elections in Scotland, Wales and Northern Ireland, and the London mayoral contest. The Remain camp needed more time to build up 'a positive, in-principle campaign' in order to prevail, she said.

A group of 40 Tory MPs called on the prime minister over the weekend to meet them and discuss his negotiating stance. John Baron MP urged voters to “ignore those who predict Armageddon should we leave”.

The drugmakers’ move is the latest sign that Cameron’s appeal to business to speak out is having an effect. John Lechleiter, chief executive of Eli Lilly, told the Financial Times that it would be a 'shame and a mistake' if Britain left the EU. The US company employs 2,800 people in the UK across four sites, including at a large drug discovery facility in Surrey, and said it ranks among the top 10 US investors in British research and development.

Lechleiter said their UK research center was “literally a global research center” and that he thought they had people there from every country in the EU. “We tend to think about our UK operation as being very much a European center.”