Campaigners representing tens of thousands of people with multiple sclerosis (MS) are to lobby the government over welfare changes they claim will restrict help for people with fluctuating conditions.

People with MS who have walking difficulties would have qualified for the enhanced rate of benefit under the old rules if they were unable to walk more than 50 metres but this has now been cut to 20 metres, according to the MS Society.

The group will "march" 20 metres and deliver an open letter with 4,000 signatures to the Department for Work and Pensions (DWP) in London highlighting the new qualification criteria for the enhanced mobility rate of the Personal Independence Payment (PIP), which starts to replace the Disability Living Allowance (DLA) from April.

"If you can walk the length of two double decker buses then you will not qualify for this payment. The problem with MS is that you may be able to do that today, but tomorrow you may not," a spokesman said.

"We are saying it is about being able to do this repeatedly and safely."

He said the change would affect the eligibility of people with MS to equipment as well as cash.

"Many will risk losing their motability vehicles," he said.

"It might well restrict their ability to live independently, for example getting to work, taking children to school, attending medical appointments, that sort of thing."

Nick Rijke, director of policy and research at the MS Society, said: "The number of people who have added their signatures to our letter shows the strength of feeling about this issue.

"Many people with MS rely on this support to help them maintain their independence - losing the money could have a devastating effect."

The MS Society, which represents 100,000 people in the UK with the neurological condition, said it expected that about 46,000 people of working age with MS will be affected by the move to PIP.

A spokesman said the latest research suggested about 600,000 disabled people overall will lose out under the changes.

Currently those qualifying for the mobility component of PIP at standard rate receive £21, with those on the enhanced rate receiving £55.25. The MS Society said being downgraded would mean a loss of £34.25 a week, or £150 a month.

A DWP spokesman said: "The higher rate mobility was always meant to be for those disabled people who are unable to walk or virtually unable to walk.

"It is not a tightening of the rules - 50 metres has never been set out in legislation.

"We had strong feedback from our consultation that the criteria was unclear, which is why we have now clarified the rules.

"Individuals who can move more than 20 metres can still receive the higher rate, if they cannot move these distances safely, reliably, repeatedly and in a reasonable time period.

"Through the changes we are making, a greater percentage will get the higher rates under the new benefit - 23% under PIP compared to 15% under DLA.

"This shows support is being focused on those who need it most."