The file photo shows British Prime Minister David Cameron addresses a press conference at the end of an extraordinary two-day EU summit at the European Council in Brussels, Belgium, Feb 19, 2016. [Photo/Xinhua]

LONDON - A government guarantee that protects pensions for millions of elderly people would be under threat if Britain quits the EU, Prime Minister David Cameron warned Sunday.

A so-called triple-lock guarantee has been government policy since 2010, ensuring an annual above-inflation rise in state pensions.

The grey-vote became the battleground in what is the last full week of campaigning ahead of a June 23 national referendum to decide if Britain should remain as a member of the European Union, or leave.

Cameron warned of a self-inflicted recession, leading to a lost decade as Britain grappled with a Brexit success.

"It would be irresponsible not to talk about the risks of leaving," he said, warning of a black hole of up to 57 billion US dollars in Britain's public finances.

In the Sunday Telegraph, Cameron said: "We'd see a decline in the value of the pound, and of the value of the assets we own, like our homes. And because we'd probably need to raise interest rates to help deal with the shock, mortgage rates would rise.

"One area we'd be forced to look at is pensions. We said: pensioners should expect dignity and security in retirement. That's why we introduced the Triple Lock. It means that basic state pension will always rise in line with whichever is higher - earnings, inflation or 2.5 per cent.

"But if we had a big black hole, we could struggle to justify this special protection any longer. Even if we could justify it morally, it wouldn't actually be affordable. Not when pensions represent a huge portion of public spending - over 90 billion pounds (128 billion US dollars) this year - and when inflation is forecast to hit 4 per cent if we leave Europe. So here is the reality: if we leave, the pensioner benefits would be under threat, and the Triple Lock could no longer be guaranteed in the long term."

Cameron's warning was roundly condemned by leave campaigners, with a number of commentators saying scare tactics could send people into the leave camp, resentful at being threatened by big-names in the remain camp.

Three Brexit-supporting MPs from Cameron's own Conservative Party, all rounded on Cameron. John Redwood, said MPs on all sides of the House of Commons would never vote for the triple-lock policy to be dropped.

"Mr Cameron's extraordinary claim that pensions might not be safe on Brexit is wrong," Redwood wrote on his social media site.

Iain Duncan Smith described Cameron's threat as baseless and vindictive, while Priti Patel accused Cameron of scaremongering and spreading fear.

The Sunday Express newspaper claimed emergency battle plans are being drawn up to try to win over "remain" voters after a series of opinion polls put Leave ahead.

The newspaper said plans will see Cameron take a back seat in the campaign after polling showed he has lost the trust of voters on the referendum issue.

Instead Labour, reported the Express, will be put in charge of winning over wavering voters in the North of England, while further efforts will be made to discredit prominent Leave supporters, including former London mayor Boris Johnson.

The Express quoted an unnamed source as saying Downing Street had been given a wake-up call and "now realised the prospect of Britain leaving the EU was very real."

The leader of the Church of England, Archbishop of Canterbury Justin Welby declared Sunday he supported Remain.

"June 23 is a date on which we, happily, do not literally have to fight for our freedom or future but we are going to make a choice that will change the lives of all of us, and the next generations, both in this country and indirectly for much of Europe," wrote Welby.