DUP finance minister Peter Robinson has told Sinn Fein that the executive will no longer pay for the party to provide its own ministerial drivers.
Ministers must use assembly drivers, the finance minister says
He said the party must now use drivers from the official assembly pool like other ministers or pay for its own.
In the past it is understood individual departments with a Sinn Fein minister paid the party for supplying drivers.
Sinn Fein's John O'Dowd said the party's drivers were more cost effective than those from the pool.
He said the practice was agreed by the previous Northern Ireland Executive and said Mr Robinson had not raised concerns about the accounting practices.
"If the executive wishes to re-examine this - and I personally believe there are much more important things to be doing - then Sinn Fein will involve themselves in the re-examination.
"It remains to be seen if Peter Robinson has the authority to do what he has done," Mr O'Dowd said.
Mr Robinson said he stopped Sinn Fein using its own drivers on 23 May as "this ensures there will be a consistent approach which confirms with best accounting practice".
The issue was first raised by DUP assembly member Jonathan Craig.
"If other government departments were paying private enterprises to deliver services which were already being delivered by a government department, I think you would find we would have to force an investigation on that matter," he said.
"I'm not determined to make it a big issue, I would just like to think that Sinn Fein will now fall into line with all the other political parties."