Disability campaigners have challenged a claim by the Prime Minister that people with a live-in carer will be exempt from the "bedroom tax".

Under UK Government welfare reforms households deemed to have one spare bedroom will have their benefit reduced by 14% of their eligible rent, while those with two spare bedrooms will face a reduction of 25%. Opponents have dubbed the spare room subsidy policy the "bedroom tax".

David Cameron said last week: "Anyone who needs to have a carer sleeping in another bedroom is exempt from the spare room subsidy."

But Motor Neurone Disease (MND) Scotland, which helps sufferers of the degenerative neurological illness, said this contradicts advice it received from the Department for Work and Pensions. An MND briefing paper on welfare reform states that sufferers often find it difficult to share a marital bed, forcing their partners to sleep in another bedroom.

It says: "MND Scotland wrote to Work and Pensions Secretary Iain Duncan Smith urging him to consider revising the scheme so that people with MND are not unjustly penalised.

"The Government response states that discretionary housing payments (DHPs), administered by local authorities, will be available to priority groups, including those who have had their homes significantly adapted.

"However, DHPs are variable depending on your local authority, short term and one off payments. They do not present a solution to people with MND who will be competing with other groups for this limited pot of money."

An MND Scotland spokeswoman said: "We call on the DWP and UK Government to exempt from the 'bedroom tax' those who have had their homes significantly adapted for their needs and/or are unable to share a bedroom due to an array of medical equipment necessary to keep them safe during the night."

SNP MSP Christina McKelvie has written to Mr Cameron urging him to clarify his comments.

She said: "David Cameron clearly doesn't understand his own policies so I am writing to him for clarification to find out exactly what he means. The last thing somebody needs when they are terminally ill is the threat of being evicted and this is exactly what the UK's bedroom tax is doing.

"The leader of the Westminster system's rhetoric stands in direct contrast to the reality of what is happening."

She added that an independent Scotland would have its own welfare policy from day one, offering a real alternative to Westminster's "cuts agenda".