The Parades Commission has said the Apprentice Boys breached its ruling on a feeder march in Co Tyrone at the weekend.
Three people were arrested on Saturday night during clashes between a small number of nationalists and police at Castlefin Park in Castlederg.
On its return from the main Lundy's Parade in Derry, the Apprentice Boys Garvetagh club assembled to march in Castlederg.
On reaching Lurganbuoy Road it was met with a line of police but when marchers produced a letter from the Parades Commission they were allowed to proceed part of the way along the road, which leads to three nationalist areas.
However, it has emerged that the letter did not specify Lurganbuoy Road and had no legal weight.
In its ruling on the disputed parade last week, the commission stated: 'The parade is prohibited from entering that part of its notified route comprising Priest's Lane, Ferguson Crescent, Killeter Road, Alexander Park and Lurganbuoy Road."
The same ruling applied last year but the parade did march along part of Lurganbuoy Road.
When marchers objected to this year's determination, a letter was sent from the Parades Commission confirming that the same restrictions as last year applied.
DUP assembly member Thomas Buchanan said he showed this letter to police in Castlederg and based on that, police allowed them onto Lurganbuoy Road.
Mr Buchanan said the Apprentice Boys had not breached any determination but had been allowed through police lines "on the strength of the letter".
A spokesman for the Parades Commission said the letter had no legal weight and the official notification clearly stated that the march could not go onto Lurganbuoy Road.
He said the commission would take their actions into consideration if and when the Apprentice Boys applied for another parade in Castlederg.
A police spokesman said a letter was shown to officers on the ground.
Based on it and a discussion with a Parades Commission observer, the police then allowed the march to proceed.
Sinn Féin councillor Charlie McHugh welcomed the Parades Commission statement that the Apprentice Boys breached the official ruling.
He claimed trouble only arose after police let the march through their lines onto Lurganbuoy Road.
Mr McHugh said people from nearby nationalist homes were surprised and confused by the police actions.