David Cameron has warned that if Britain leaves the EU it could put the state pension at risk as the strain on public finances caused by a Brexit would mean cuts would have to be made to "things that people really value".

Spelling out the so-called "cold reality" of opting to leave in an interview with The Observer the Prime Minister indicated Britain would be left with "a black hole" in public finances after a Brexit that may threaten the end of the "triple lock" guarantee that ensures OAP payments increases annually.

The Leave campaign has dismissed the claims as an attempt to "bully and frighten" the British public.

Cameron told The Observer: "You would have to start cutting things that people really value, whether it is the money going to the NHS or whether it is the support for our pension system, and that could mean reviewing the triple-lock."

"If we leave the EU, independent and respected experts like the IFS and the National Institute for Economic and Social Research say that by 2020 we will face a black hole in our public finances of up to £40 billion", he added.

In his warning, which comes less that two weeks before the public goes to the polls, the PM added that the NHS and defence spending also cannot be guaranteed in the event of an EU exit.

The PM's claims have been dismissed by members of the Leave campaign with pro-Brexit ex-Cabinet minister Iain Duncan Smith accusing his party leader of a "vindictive and desperate attempt to bully and frighten the British people".

Reacting to Cameron's comments he said: "This is a baseless threat.

"Experts have shown that leaving the EU - and introducing an Australian-style points system - would lead to a higher state pension.

"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."