The group 38 Degrees said a leaked draft plan suggested the cuts would be made at Leeds Teaching Hospital Trust.
It also claimed a “major reconfiguration” of frontline services in Wakefield, North Kirklees, Calderdale and Huddersfield would take place.
However Leeds hospital chiefs said the potential reduction in beds referred to the number of patients currently affected by “bed-blocking” as well as moves to cut avoidable hospital admissions.
Earlier this year health leaders across the country were tasked with planning how to deliver a “sustainable and transformed” health service which will improve quality of care and NHS finances.
In all, 44 draft plans are being prepared to cover the whole of England.
Laura Townshend, director of 38 Degrees, said the proposals to close some services show the NHS is “dangerously under-funded”.
The plans are due to be signed off in October “yet we are being kept in the dark,” she said.
She added: “The health service is struggling to cope with growing black holes in NHS funding. These new revelations will be a test of Theresa May’s commitment to a fully-funded National Health Service.”
Chris Hopson, chief executive of NHS Providers, which represents frontline NHS leaders, called on Health Secretary Jeremy Hunt and NHS England boss Simon Stevens to admit a disparity exists between what the NHS is being asked to achieve and the money available.
Simon Neville, director of strategy and planning at Leeds Teaching Hospitals NHS Trust, said the city’s health bodies agreed there were too many patients in hospital beds who did not require that level of care.
“Currently this means hospital beds are not available for other patients who need them, disrupting planned admissions and operations. To try and manage this we have been putting in place extra unplanned beds, which is expensive and unsustainable.
“The plans being developed with our NHS partners, the local authority, social care and others should help address this problem so that patients can be cared for more appropriately in alternative settings.”
He said that on average 100 patients well enough to be discharged were waiting for services outside of hospital and it was reducing this number as well as cutting avoidable admissions that was the “equivalent of five wards” mentioned in the report.
A spokesman for NHS England said: “Proposals are at a draft stage but we expect all local leaders to be talking to the public and stakeholders regularly – it is vital that people are able to shape the future of their local services. No changes to the services people currently receive will be made without local engagement and, where required, consultation.”