Theresa May will not reveal at what level the Conservatives’ social care policy will be capped before the election, members of her cabinet have admitted.

The Tories announced there would be a cap to ease the burden of the “dementia tax” after the policy triggered a backlash. But Amber Rudd, the UK home secretary, said the party was not sure about the maximum amount people should contribute towards their care costs – and that a variety of options would be considered after the election.

Asked by the BBC’s Andrew Marr whether that meant the cap could be as high as £200,000 or £300,000, and therefore not provide much reassurance to pensioners nervous about the new system, she said: “I think that what people should realise is that we know that people are living longer.

“The next decade there’s going to be another 2 million people over 75, which is great news, but we have to be frank that this is going to cost money. We have to find a way that is fair for people to pay for it. This is the best route to do it.”

May used the Conservative manifesto to introduce a means test for the winter fuel allowance and say people’s properties would be taken into consideration for the first time when deciding how much they should be for social care at home.

Despite a £100,000 floor to protect assets, the policy triggered an uproar, leading the prime minister to promise a new cap on overall costs. However, she would not provide a level for the cap when asked about it during a press conference at the G7 in Sicily.

The defence secretary, Michael Fallon, also failed to provide further details about the policy on Sunday morning. He talked about new “safeguards” when questioned by ITV’s Robert Peston but did not give a level for the cap.

Fallon also dismissed as “Westminster tittle-tattle” the suggestion that May’s joint chiefs of staff – Fiona Hill and Nick Timothy – were divided over the issue.

He then criticised Jeremy Corbyn by claiming the Labour leader was “soft on terrorism”. After Rudd used a newspaper interview to suggest there would be more terror attacks under a Labour government, Fallon also piled in.

“This is a man who has opposed every piece of anti-terrorist legislation, who does not support our military action in Iraq and Syria – where the RAF are trying to keep us safe, dealing with terrorists who are planning attacks against western Europe and indeed this country,” he said. “And he has always questioned our nuclear deterrent and indeed our Nato deployments. We would certainly be less safe if Jeremy Corbyn were our leader.”

Lib Dem Norman Lamb, who was a health minister under the coalition government, said the Tories failure to set out the cap level was “holding the public in contempt”.

“Theresa May must stop flip-flopping and do what her party had committed to, implement a £72,000 cap on care,” he said. “Conservative plans for a dementia tax are cruel and not properly thought through. It shows Theresa May is taking people for granted.”