The Duke of Devonshire has been asked by Minister of State for the Marine John Browne to remove the Clondulane weir on the River Blackwater near Fermoy, Co Cork.
The move follows a report by department engineers which says the weir at Careysville is hampering the passage of migrating fish.
The duke, a major landowner in Munster and proprietor of Lismore Castle, Co Waterford, is said to be dismayed at the move.
His agent, Michael Penruddock, said yesterday it threatened the future of one of Ireland's most renowned salmon fisheries.
In a letter sent on July 27th, Mr Browne asked for the "complete removal" of the weir by the end of the year. It asks the duke to "provide a timetable for completion" of the works.
The duke owns the Clondulane weir and fishing rights on a 1¾- mile stretch of the river at Careysville, an 81-hectare (200-acre) property acquired by his late father after the second World War.
Anglers pay an average of €3,000 a week to stay at a luxurious guest house owned by the duke on the river to avail of the salmon fishery, which employs 10 people.
Blackwater angling sources say the removal of the weir would be "a disaster" for Careysville.
Mr Penruddock said yesterday that "fish can get through at all times" and that removing the weir would destroy a "safe-haven" of deep water upstream in which salmon rest and spawn.
He said he found it "extraordinary" that the fishery was not consulted about the report and said the duke would be "seeking advice" before replying to the Minister's letter.
When contacted by The Irish Times, a spokesperson for the Minister said that the department would be happy to meet the duke or his representative to discuss the matter.
Patrick Devennie, who works for the Fort William fishery, said the environmental and ecological consequences of removing such a large structure from the river should be carefully assessed.
He said the removal of the weir would lead to "flash flooding and damage to farmland and riverbanks" and that the Minister's decision was "badly thought-out, rushed, underhand and doesn't smell right".
However, Mr Browne has received the backing of the Southern Regional Fisheries Board which believes that weirs are "redundant".
In another case, Mr Browne has also written to Fermoy council asking that a weir close to the bridge in the town, also on the Blackwater, be lowered significantly and that a rock ramp pass be installed to enable the free passage of fish.
Mr Browne said the EU Commission has received a complaint claiming that the Fermoy weir was acting as a barrier to migrating salmon and lamprey.
It will meet next month to decide if infringement proceedings should issue against Ireland which could result in "very substantial fines".
