DUP leader Gavin Robinson has called for the “full picture” over any potential threats to police officers or Orangemen, which could have contributed to the decision to ban the Drumcree parade, to be made known.
In his weekly email to party members, Mr Robinson said “transparency is key to understanding the decisions taken”.
Northern Ireland Secretary Sir Chris Bryant stepped in a week ago to ban the Orange Order march from passing along the mainly nationalist Garvaghy Road, after a request from PSNI Chief Constable Jon Boutcher to review the Parades Commission determination.
Crowds of protesters had turned out on the road in opposition to the parade, which had been permitted for the first time for almost 30 years, leading to a stand-off which lasted almost two weeks.
A court challenge to Sir Chris’s intervention is due to be heard at Belfast High Court on Monday.
A previous hearing was delayed after the challenge was broadened to also challenge the role played by the chief constable.
In his message, Mr Robinson said: “If the threat of terror was serious enough to halt a lawful parade, people should be told.
“If it was not, they should be told that too.”
The DUP leader said powers exercised in the decision under the Public Processions Act had never been used before.
He added: “Without an explanation, that is incomprehensible.
“Mob rule and threat cannot be seen to get their way. If they do, the lesson will not stay in Portadown.
“My colleagues in Portadown have lived and breathed this issue for two years.
“They have every right to voice the anger of the people they represent, and I stand with them.
“Like me, they have pressed the chief constable to be open with the public.
“As this returns to court on Monday, the chief constable must set out the full picture of the dissident threat to his officers and to Portadown district.
“Transparency is key to understanding the decisions taken.”
Mr Boutcher previously said there had been no disorder amid the two-week stand-off, which he added “should be viewed positively by everyone”.
He added: “It was my ongoing assessment that short of a major public order operation, that would require substantial numbers of protected officers and specialist capabilities with a likelihood of prolonged disorder resulting in injuries to the public and officers, with an impact beyond Portadown, the parade could not be safely undertaken.”
Meanwhile, Mr Robinson said a PSNI investigation into potential offences during protests on the Garvaghy Road “must run its full course”.
A number of Sinn Féin elected representatives, including First Minister Michelle O’Neill, have received letters from police inviting them for interview.
Mr Robinson said: “The chief constable must keep the public regularly updated on its progress.
“The same transparency demanded on the threat picture must apply here too.”
Ms O’Neill has said she would co-operate with the police investigation and defended standing “in solidarity with the Garvaghy Road residents as attempts were made to force an unwanted Orange march through their community”.