Disabled activists are planning legal action over the government's decision to tighten eligibility for support for people with the highest mobility needs.

Many disabled activists were shocked and angry when Esther McVey, the Conservative minister for disabled people, announced the unexpected changes in December.

The alterations to the regulations that will decide who is eligible for the new personal independence payment (PIP) - which will gradually replace working-age disability living allowance from this April - saw the key walking distance criteria reduced from 50 to 20 metres.

Lawyers advising the WeAreSpartacus online network of disabled campaigners have now suggested that this lowering of the distance criteria could be unlawful.

Jane Young, coordinator of WeAreSpartacus, said: "The reason for a legal challenge and a great part of its basis in law is that there was absolutely no consultation on 20 metres at all.

"One of the reasons people are particularly angry about this is that the 20 metres has literally come out of nowhere.

"We realised PIP would not be great, but it wasn't until the regulations were published that we realised how bad it would be from the mobility perspective."

She is now appealing on her blog for disabled people - although only those in certain categories - to come forward to take a case against the Department for Work and Pensions (DWP).

Earlier this week, McVey insisted again that the "moving around" part of the PIP assessment - which includes the 50 to 20 metres change - had not been tightened in December.

She told a committee of MPs debating the PIP regulations: "I know that there have been concerns that we have tightened up rules regarding the physical mobility activity 'moving around'. I take this opportunity to reassure honourable members that that is not the case."

This "reassurance" came despite the government's own figures making it clear that the overall package of changes announced in December would see - by 2015 - 20,000 fewer people eligible for the enhanced mobility rate than under the previous draft version of the PIP regulations, with this gap rising to 51,000 by 2018.

McVey claimed that, under previous drafts, the government had "differentiated by the type of aid and appliance that an individual used", and "used terms such as 'up to' certain distances, which meant it was not clear which descriptor applied to people".

She said: "In the final version of the criteria, we differentiate by distance, which we feel is much clearer.

"In the final draft, individuals who cannot walk 20 metres can be certain they will receive the enhanced rate, regardless of whether they need an aid or appliance."

And she insisted that last month's decision to ensure that the need for activities to be carried out "safely, reliably, repeatedly and in a timely manner" was included in the regulations would make "the protection in our policy stronger".

But Anne McGuire, Labour's shadow minister for disabled people, said disabled people had "been alarmed at the hurdle they will now face before being awarded the enhanced mobility rate".

She said: "Most of those who have looked at these descriptors would say that a 20-metre qualifying distance simply does not provide a practical level of mobility."

She said about 200 disabled people in every parliamentary constituency were likely to lose their Motability vehicle with a 20-metre qualifying distance.

And she pointed out that one of the Department for Transport's own publications, Inclusive Mobility, recommends that "seating should be provided on pedestrian routes at intervals of no more than 50 metres, and that parking spaces for blue badge holders should preferably be provided within 50 metres of the facilities they serve".