The highly unusual move by John Prescott - the Deputy Prime Minister and Secretary of State for Environment, Transport and the Regions - has been sparked by the possibility that the Ministry of Defence may double the amount of exercise time that it said it would need for the 45-tonne AS90 howitzer.
The gun, the army's most important piece of field artillery for the next 25 years, is very bulky - it looks like an outsize cannon on a tank - and requires wide roads and a very great deal of support.
To train just one regiment in Northumberland the MoD wanted a pounds 50m upgrade of the Otterburn training area, which would include 50km of new and improved roads, 46 stone gun platforms as well as a central depot that would be the size of six football pitches.
Environmentalists and the National Park Authority vigorously opposed the plan, saying that the developments would unacceptably scar the landscape, and that vehicle movements and noise would impair seriously the park's tranquillity and peace.
An AS90 regiment of 24 guns needs more than 300 support vehicles.
The inquiry lasted from April to October last year and the inspector's completed report has been on Mr Prescott's desk since the early summer.
But the possibility has since arisen that not one, but two AS90 regiments may be heading the park's way.
This summer's strategic defence review announced the formation of a new UK-based regiment, and the only other place suitable for its training, Salisbury Plain, is, by the army's own admission, full.
Fearing that the additional guns must be bound for Otterburn - the army is non-committal about it - the National Park Authority and the Council for National Parks, the pressure group, asked Mr Prescott to reopen the inquiry to have the additional burden on the park properly considered.
Now he has agreed with them, and an official letter to those taking part says he is "... minded to reopen the inquiry in order that this new material can be considered."
"We are delighted," said Emma Loat, policy officer for the CNP.
"There is unequivocal evidence that an extra regiment of AS90s is coming to Britain, and, based on what the army says about Salisbury Plain, the only other place they could train would be Otterburn. This needs to be properly considered by the public inquiry.
"It will be bad enough if such a peaceful protected area is invaded by one regiment of these guns and their support vehicles, but an extra regiment would be totally unacceptable."
Northumberland is the remotest and one of the least visited of the ten English and Welsh national parks, with sweeping landscapes of moorland over two-thirds of its area, which provide those who want it with real solitude.
It is also the home of England's two rarest mammals - the pine marten and the rapidly declining red squirrel. The park is the red squirrel's last real bastion.
The army told the enquiry it accepted that the proposed new level of training - for one regiment - would reach the limit of what was acceptable in the park.
The then range officer for Otterburn, Lieutenant Colonel Richard Cross, said that "any more than the artillery training proposed by this [planning application] would be unacceptable, and should seriously be reviewed by Land Command."Reuse content