The proposals, made by Defra, intend to reduce the level of elected representation on the body, which covers the west part of the county from the Tweed to the Tyne.
At present, 12 of the 22 seats on the Authority are filled by local councillors.
Six are from Northumberland County Council, one from Berwick Borough, two from Alnwick District and three from Tynedale District.
Four members represent the parishes which fall inside the National Park and the remaining six places are Secretary of State appointees.
But under the new layout, just six seats would be filled by elected councillors from the unitary authority for Northumberland.
Of the remaining 14 seats, six would be filled by representatives for the parish councils, increasing their representation from four seats to six.
The number of people appointed by the Secretary of State would rise from six to ten.
North Northumberland MP, Sir Alan Beith, slammed the proposals and said the Government seemed intent on dominating local decision-making.
"We are now being told that not only do we have a single unitary authority imposed on us but also that one effect of the reorganisation is that central government can boost its number of representatives on the National Park Authority at the expense of local representation," he said.
"Given that the Northumberland County Council has to make sure that the different parts of the National Park still need to be represented, I am sure the council will need more than six places in order to do so, because it has to cover from the Scottish Borders down to Tynedale and the many different communities in the area.
"This move confirms many of our fears that local government reorganisation is being used to increase central government power and reduce the power of people living in the area affected."
Sir Alan has now written to the Secretary of State about the proposals.
County councillor for Rothbury, Steven Bridgett - who is an authority member - said: "The Government is desperate to cling on to power by whatever means possible.
"It's deplorable."
