The DUP has declined to comment on claims it fines its politicians if they talk to the media without permission.
A DUP insider told the BBC’s Nolan Show politicians are forced to pay as much as £1,000 if they break internal protocol on dealing with the press.
The DUP said it operated under a constitution and a code of conduct, passed by its executive.
The BBC has seen a letter signed by the DUP’s chief executive imposing a fine on an elected representative.
While the politician was asked to pay a £100 fine, the BBC understands repeat offenders might face higher charges of £500 or £1,000.
The other main Stormont parties all told the BBC they do not impose similar penalties.
The practice of fining elected representatives was “quite unacceptable”, said the former chairman of the Committee on Standards in Public Life, as it undermines democracy, and is authoritarian and draconian.
“Elected representatives represent their constituents, even though they have been elected on a particular party ticket, they are there to put over their point of view representing those constituents,” said Sir Alastair Graham.
“If they are severely constrained through threats and fines, then that really does undermine their effectiveness,” he added.
Analysis: Mark Devenport, BBC News NI political editor
The latest revelations emphasise the DUP’s tight internal discipline.
During the early years of the peace process, Ian Paisley’s party chipped away at David Trimble’s Ulster Unionists.
The DUP was happy to see its rivals fracture into different camps, but determined to avoid the same fate.
In 2007, as the DUP contemplated sharing power with Sinn Fein, the BBC reported that party candidates were being asked to sign post dated resignation letters and to pay big fines if they stepped out of line.
The Ulster Unionists described the DUP rules as bordering on “fascism”.
The signature on the letters imposing fines is that of Timothy Johnson.
The DUP press officer – who declined to comment on the internal management of the party more than a decade ago – has now risen through the ranks to be the party’s chief executive.
The letters emphasise that all DUP “these rules apply to all MPs/MLAs/councillors on political matters that are outside of their narrow constituency business”.
Reporters working the Stormont beat will not be surprised at the sanctions facing individual DUP politicians, which explains some of the furtive nature of our interactions.
But over in Westminster our colleagues, used to chatting with Conservative and Labour rebels as they decide whether or not to break ranks, may be surprised to hear that such a blatant system of fines is in place within the ranks of Theresa May’s crucial parliamentary allies.
It said the party’s chief executive, Timothy Johnston, writes to members when required “from time to time”.
But the DUP has not commented on its internal procedures.
“The party reserves the right to decide what program to participate in, and the best mix of platforms to ensure communications with the people of Northern Ireland,” it said in its statement.
The allegation that the DUP impose fines came to light in an interview given by an anonymous party representative to the Nolan Show.
“It was Timothy [Johnson] who sent me the letter,” he said.
“He emailed me and then he sent me a letter saying: ‘You’ve spoke to the BBC (which I did) and we’re fining you £100’.
“There’s been lots of people fined. It’s quite a frequent occurrence.
“The press office would report me to Timmy Johnson and Timmy Johnson then makes a decision whether or not to fine the person.”
The representative said that Mr Johnson acted as “God incarnate” within the DUP, saying he was “judge, jury, executioner, implementer, enforcer, advisor”.
The representative also said that the DUP is officially boycotting the the Nolan Show due to its coverage of the RHI scandal and that the party tightly controls what MLAs are permitted to say during all media appearances.