Home Secretary Amber Rudd has said she does not know what the proposed cap on the social care costs would be in her party’s controversial plans to make elderly people pay for their own social care, dubbed the “dementia tax”.

When the policy was announced in the Conservative manifesto there was no upper limit on how much elderly people might be forced to pay for their care, only that they would not have to pay any more once the value of any estate that remained to leave to their children had been reduced to £100,000.

In the wake of criticism, Theresa May then announced there would be a cap, but elderly voters – who vote Conservative in overwhelmingly large numbers – will now be expected to go to the polls with no idea about what that cap might be.

Ms Rudd told the BBC’s The Andrew Marr Show: “The Prime Minister has said yes, there will be a cap, but we are not sure where the cap will be.

“What we are saying is we will have a green paper to make sure that we set it at the right level and we consider all the other alternatives.”

Ms Rudd also revealed that new powers to temporarily exclude suspected jihadis from returning to the UK have been used for the first time.

Temporary exclusion orders make it unlawful for the subject to come back without engaging with UK authorities.

Ms Rudd would not say how many times TEOs had been used, but confirmed "we have started to use them". The last time figures on the use of TEOs were made public the number that had been used was ‘zero.’

The Home Secretary also admitted the authorities do not know how many Britons had returned from fighting with Islamic State or other extremists in Syria.

Manchester suicide bomber Salman Abedi is reported to have travelled to Libya in 2011 during school holidays to join his father Ramadan in the fight against Muammar Gaddafi, and it has also been suggested Abedi "most likely" went to Syria but Turkish officials reportedly said they have no record of him travelling to the war-torn country.

Asked how many people were thought to have returned from the war-torn Middle East state, Ms Rudd said: "We don't know the exact number.

"What we do know, in engaging with the intelligence services and with the police and with the Border Force, we make sure that they have the tools to track them and to keep them out where we can."

The Home Secretary also said "good progress" was being made with online firms about finding ways to tackle terrorists using secure communication services such as Whatsapp.

"We are making good progress with the companies who have put in place end-to-end encryption," she said.

"Some of them are being more constructive than others and we will continue to build on that.

"The area that I am most concerned about is the internet companies who are continuing to publish the hate publications, the hate material that is contributing to radicalising people in this country."

She added: "What we are doing is challenging the people who are delivering end-to-end encryption to work with us so that we have a way of keeping people safe.

"Nobody wants terrorists to have a safe place to exchange information and to be able to plot their terrible atrocities.