The NHS could be put under further pressure if the UK broke away from the EU, Tim Farron has suggested. The Liberal Democrat warned that elderly ex-pats, forced to return to Britain after a split from Brussels, would place a significant strain on public services.

The pro-EU politician made the claim after launching the Liberal Democrats' In Together campaign on 10 February in London, where IBTimes UK pushed the Westmorland and Lonsdale MP on the issue of immigration.

"I have to ask the question, given the hundreds of thousands of retired British people living in Europe, how our NHS would cope when they have to return? So it's right that we understand that there are infrastructure pressures in both directions," Farron argued.

The Liberal Democrat leader also stressed that it was "very reasonable" for British voters to have concerns about immigration, but Farron said the free movement of people was good for the UK economy and diversity. "By-in-large, immigration levels will balance themselves out [across the EU]," he added.

There are 1.26 million UK citizens living in other EU member states, according to the most recent official data. That figure equates to just 0.3% of the political and economic union's population (excluding the UK).

Farron's comments came after he tore into David Cameron's claim that makeshift camps like the "Jungle" in Calais could appear in England after a Brexit. The Liberal Democrat described the prime minister's remarks as "disgusting. [Cameron] has shown himself to be weak, and heartless. And this campaign needs the opposite. This campaign needs strength and compassion," Farron declared.

"The leave campaign will play nasty, and it seems people on our own side will engage in a nasty race to the bottom on immigration, migration and refugees. But Liberal Democrats, I will not stand for it."

The Liberal Democrat leader also predicted that the refugee crisis across Europe would "escalate", but he argued it would be imperative for the UK to stay inside the EU to help solve the humanitarian situation.

"Europe is not perfect. Westminster is not perfect. But picking up your ball in a sulk and heading home is not the way to win," Farron added. "We should be a leading voice to make changes. Remaining in a reformed Europe. This is Britain's time to lead the way."

On the other side of the EU debate, Ukip has launched an attack video against Cameron's reform efforts.The Eurosceptic party mocked the prime minister's "emergency migration break" as an "emergency fake". The proposed measure would see the UK apply to Brussels in a bid to stop giving in-work benefits to EU migrants inside the UK.

Cameron is yet to set a date for the EU referendum as he wraps up his renegotiation. But the vote is expected to be held on the 23 June. The latest opinion poll from YouGov for The Times, of more than 1,600 people between 3 and 4 February, put the leave vote nine points ahead of remain, at 45% versus 36% respectively.