As a backdrop for Project Fear it could scarcely be bettered: a Eurofighter Typhoon fighter jet in the final stages of assembly, primed for sale to a friendly dictatorship in the Middle East.

Before the warplane, in a vast hangar near Preston, stood British prime minister David Cameron, surrounded by staff at the BAE Systems plant where the aircraft is assembled.

Cameron had come to the northwest to tell defence industry workers why they should vote to remain in the EU – and to warn them of the threat to the economy and to national security if they don't. To say nothing of the fact that they might lose their jobs.

"There are three million jobs that in some way depend on our trade in the European Union, " Cameron said. "Of course we would go on trading with the EU if we left, but would that trade be at the same level? How many of those jobs would truly be safe? Would we be able to build on those jobs if we had a massive uncertainty about our future economic relations?"

Sales agreed

Around the hangar stood fighter jets close to completion, the canopy open above a mustard- coloured fuselage, grey nose pointing downwards. They will soon be delivered to their new owners, their sale itself the fruit of co-operation, as European partners divvy up the despots in the desert kingdoms of the Gulf.

"We spend a lot of time trying to work out who is best- placed to win these export orders," the prime minister said. "We've got hopefully good news coming from Kuwait. The Italians have been doing a lot of work there. The British have been working very hard in Oman. I can see the planes being built right behind me here. We've got more work to do in Saudi Arabia. The Germans have done a lot of work as well. It is a collaborative project."

Cameron has been slapping down Conservative colleagues all week, beginning with Boris Johnson, whom he humiliated in the House of Commons with a blistering assault on the London mayor's integrity.

On Wednesday, it was the turn of justice secretary Michael Gove, who had questioned the legal status of the deal agreed last week in Brussels.

On Thursday it was work and pensions secretary Iain Duncan Smith, who had claimed that Cameron's renegotiation would do nothing to reduce the number of EU migrants coming to Britain.

The prime minister told his audience in Preston that the welfare curbs he secured would indeed reduce migrant numbers. And he warned that leaving the EU could actually cause the numbers to increase.

Free movement

“The countries outside the EU that have full access to the single market, like

Norway

, they have to accept the free movement of people,” Cameron said.

“In fact, if we left the EU the deal that I’ve just negotiated doesn’t stand. So we actually have to accept free movement if we’re in the same position as Norway, and we wouldn’t have the welfare restrictions that I’ve just negotiated.”

Every question from the audience offered Cameron an opportunity to identify a new risk involved in leaving the EU, from more expensive air travel and higher supermarket prices to encouraging Vladimir Putin to invade another European country and allowing pirates to roam freely off the coast of Somalia.

Among the beneficiaries of a vote to leave the EU, he said, would be terrorists and criminals seeking to escape across international borders.

"When those terrorists tried to bomb London in 2005, the second time – the 21st of July – one of the bombers escaped out of the UK, headed off to continental Europe and because we are part of the European arrest warrant he was brought back within weeks and is now sitting in a British jail serving a 40-year jail sentence.

“Before that European arrest warrant it would take years, sometimes decades, to get a criminal out of a European country and back into Britain,” the prime minister said.

After an uncertain start following his return from Brussels, Cameron has hit his stride, hammering home the same message everywhere he goes: that leaving the EU represents a dangerous leap in the dark.

As a notice on the wall behind him in Preston put it “Think Safety First – Everyone’s Responsibility”.