DAVID Cameron has defended his claim that pensions could be put at risk if Britain leaves the EU after he was accused of attempting to bully and frighten elderly voters by prominent members of his own party.

The Prime Minister has said leaving the EU would damage the UK economy and force the Government to implement tax rises, public spending cuts or increase borrowing, highlighting a pensions "triple lock", a key Tory pledge that sees the state pension rise by at least 2.5 per cent a year, as one policy that could face the axe post-Brexit.

He also warned that other pensioner benefits and NHS funding could be squeezed should the UK quit the EU, sparking angry claims from the Leave campaign that the Prime Minister was ramping up 'project fear' rhetoric out of desperation, with polls suggesting a consistent lead for the Remain side over recent months has evaporated.

In an escalation of the 'blue-on-blue' attacks that have been a feature of the campaign, pro-Brexit ex-Cabinet minister Iain Duncan Smith accused his party leader of a "vindictive and desperate attempt to bully and frighten the British people".

"This is a baseless threat," he said. "What we now have is a vindictive and desperate attempt to bully and frighten the British people - particularly pensioners - all in a frantic bid to rescue a failing campaign."

Meanwhile, Priti Patel, the employment minister, described the pensions warning as "fear on steroids". She added: "We've had months and months of regular fear and now this has been amplified. Effectively the Prime Minister is undermining our own manifesto promise on pensions, the triple lock and the very points that Dave was making in terms of protecting bus passes, TV licences, all the things that I campaigned for in the general election.

"I simply think this is just an unrealistic statement, another intervention which quite frankly has been there to terrify pensioners in a very demeaning way. I think our pensioners are more intelligent than that. They can make up their own minds."

Nigel Farage, the Ukip leader, downplayed the economic implications of victory for Vote Leave, suggesting some short-term turbulence could be beneficial and dismissing claims the pound was already under pressure.

"Since Brexit became a possibility, sterling is up and FTSE is exactly the same level it was in March. These are ludicrous scare stories that are being put out," he told BBC1's Andrew Marr Show.

"Even if Sterling were to fall a few percentage points after a Brexit, so what? The point is we have a floating currency and it would be good for exports."

However, Mr Cameron insisted he had a duty to spell out the economic risks of leaving the EU, warning that Brexit would leave a "black hole" in the public finances, a position he said had been backed up by independent forecasters.

He also hit back at Mr Farage for dismissing the risks of a fall in the value of sterling. The Prime Minister said: "I'll tell you 'so what'. If the pound falls then that means the prices in our shops go up, the weekly shop costs people more, that family holiday costs more. These are all risks we should avoid and we shouldn't risk it.

"I totally accept that people are confused by having so many statistics and that there is a lot of frustration because of that. But I think it is actually my job as Prime Minister, when you have got information coming from independent forecasters ... to talk about those risks and it would be very irresponsible not to."

The Remain camp has sought to press home its central message of likely economic turmoil in the event of a vote to quit the bloc on June 23 amid signs of a hardening of public opinion against the EU. The Leave campaign is expected to focus on immigration in the final days of the campaign.

John Curtice, the professor of politics at Strathclyde University who is widely seen as the UK's top polling expert, said the UK's fate could hinge on which of the two issues - immigration or fears over the post-Brexit economy - prove most influential.

He said: "Remain still have some potential aces up their sleeve. As in the Scottish referendum, some voters may swing back to the status quo at the last minute as they fear the risks of change. Remain's potential Achilles heel is immigration. Many who back Remain suspect immigration would go up - and are non to enamoured with the prospect."