The head of Muscular Dystrophy UK, which represents more than 110,000 children and adults in the UK living with one of over 60 muscle wasting and weakening conditions, said people in dire need of additional support are being denied "life-changing equipment", and called on the Government to act urgently.
New figures from NHS England show 71 people assessed as needing a wheelchair by the NHS Cornwall and the Isles of Scilly Integrated Care Board in the three months to June had to wait longer than the national target of 18 weeks to receive one.
This was less than the 82 who waited over the target in the same period a year earlier.
Meanwhile, the number who did receive a wheelchair within the target period also fell slightly, from 110 last year to 102.
Across England, 7,284 patients did not receive their required wheelchair within the target period, a 21% increase on the 6,022 last year.
And the number who did receive one fell from 30,754 to 28,324.
Of the people who did have to wait over 18 weeks in Cornwall and the Isles of Scilly, 10 were assessed to have the most severe 'specialist need', and 12 were children.
Muscular Dystrophy UK manages the Joseph Patrick Trust, which provides grants towards the costs of powered mobility equipment, which can cost up to £30,000 each.
Its chief executive Andy Fletcher said: "It’s disappointing that people who desperately need wheelchairs are having to wait even longer to access life-changing equipment.
"As a charity, we know the needs of people living with muscle wasting and weakening conditions are not being met.
"This is evident from the figures and our helpline has seen an increase in calls from those struggling to access appropriate equipment.
"The NHS must take urgent action to ensure everyone can access the right equipment when they need it."
NHS England was approached for comment.
For more information about the Joseph Patrick Trust call the charity's free helpline on 0800 652 6352 or visit musculardystrophyuk.org.