Campaigners said that people with the disease are “running out of time” as the cash is held back by “red tape and bureaucracy”.
The concern over the whereabouts of the cash has intensified after the death on Saturday of former Scotland international Doddie Weir following a six-year battle with the disease.
Health secretary Steve Barclay insisted the delay was due to the “quality of the research bids” and the money “is there”.
But the Motor Neurone Disease Association said none of the money was in the hands of researchers. And rugby star Kevin Sinfield said he “can’t see any reason” why scientists cannot access the cash.
The former Leeds Rhinos player has raised millions for MND charities after being inspired by Weir, as well as his former Leeds Rhinos team-mate Rob Burrow and ex-Bradford City captain Stephen Darby, who both live with MND.
He told BBC Breakfast: “The most important thing for someone who has been diagnosed is time and they’re running out of time.
“I can’t see any excuse or reason why that money has been held back.
“And if we’re going to help them make a difference and shift the dial on this disease, that money needs to be available sooner rather than later.”
Burrow also accused ministers of holding back the cash. In a Twitter post on Saturday, he wrote: “So sad to hear the news of the passing of my MND hero Doddie Weir.
“I’m sorry to say, how many more warriors die before this stupid government give the 50m they said they would give.”
In November last year, the Government committed at least £50m to help find new therapies, and eventually a cure, for MND, a condition in which the brain and nerves progressively degenerate.
On a visit to the Royal Marsden hospital in London, Mr Barclay said: “The funding is there, we have that funding, we’re ready to allocate it.
“The delays so far has been in terms of the quality of research bids, that has come in on MMD – it’s not an area traditionally that has had a huge amount of medical research activity.
“So we’re working with the MND community to find the best way of getting that money allocated. It’s not an issue of finance, the money’s available, it’s ready to allocate.
“At the same time, we want to make sure that it goes on the right research and that’s what the chief scientific adviser within the Department of Health – Professor Lucy Chappell – is actively engaged on.”
Dr Brian Dickie, director of research development at the Motor Neurone Disease Association, said: “We share the frustrations of so many within the MND community that one year on from the promise of funds for targeted MND research – £50m over five years – none of it is in the hands of researchers.
“We have had reassurances from the top of Government that the committed funds will be delivered in full – it’s developing the mechanism for researchers to access the money in the most effective and efficient way that is causing the delay.”