RSN chief executive Graham Biggs issued the warning as he gave evidence to a House of Lords inquiry investigating whether rural communities are being left behind by government.
Peers on the natural environment and rural communities select committee are focusing on ways government policy could change to better support rural needs.
Since the demise of the Commission for Rural Communities, there had been an absence of adequate in-depth research into rural issues, said Mr Biggs.
The commission – an independent government body which succeeded the Countryside Agency, also acted as a rural advocate and watchdog until its abolition in 2013.
Mr Biggs said: “I don't think anyone is fulfilling the watchdog role as such.
“I think in terms of the advocate role, many individual organisations – and indeed collectives, like the Rural Coalition – can do a very good job.
“But I think if you are going to be the watchdog, you have to have a body that understands the whole of that role and is accepted across Whitehall, as having the right and responsibility to exercise that function – and that its views will be listened too.”
Another commission role was to push for the “rural proofing” of government policies to ensure ministers took into account the needs of rural communities.
Mr Biggs said he had thought long and hard about the issue, adding: “I really, really struggle to find where rural proofing has been an undoubted success.”
Rural-proofing was almost entirely considered too late by government to do the job needed, saidd Mr Biggs.
A Department of Education policy that all schools should provide school meals, for example, had proved a particular challenge for rural schools.
It was an entirely laudable policy but completely impractical in many smaller rural schools which didn't always have the necessary facilities to comply.
During the same evidence session, the committee also heard from Sue Chalkley, chief executive of the Hastoe Housing Association.
The commission was badly missed, she suggested.
Ms Chalkley told peers: “Since it has gone, we haven't had the same quantity and quality of data around rural communities.”
There was a need for a body which communicated what rural communities were really like – and challenged the view that they were similar to urban communities but smaller.
Earlier, the committee heard evidence from Margaret Clark, chair of the Rural Coalition; and Jeremy Leggett, a trustee of Action with Communities in Rural England.
Ms Clark said the decision to get rid of the Commission for Rural Communities had probably led to the rural voice being diminished, overlooked and often undervalued.
“The role of a rural watchdog has disappeared,” she said. “There is nobody holding, not just government, but other bodies to account.”
Mr Leggett said successive governments had lacked the political will to ensure that their policies didn't have a detrimental affect on rural communities.
“If you don’t do that early enough in the process of developing policy and then the mechanisms for delivering policy, you will always be blind to the rural dimension,” he said.
The evidence session can be watched on Parliament TV here. Graham Biggs starts giving evidence at 11.58am.
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