A former DUP mayor of Coleraine has admitted six charges of electoral fraud.
The charges relate to the last general and local government elections in May 2005, when Mr Stewart was elected to the post.
The councillor initially denied the charges, but on Wednesday, he admitted to four counts of pretending to be someone else in order to cast postal votes, as well as two charges of fraudulently stopping free exercise of a proxy vote.
He will now face sentencing on October 24.
Ian Paisley’s DUP routinely accuses Sinn Fein of electoral fraud, but always without evidence. The party is thought to be extremely embarrassed by this week’s admission.
Both Sinn Fein and the SDLP have called for Mr Stewart to resign from the council.
Sinn Fein councillor Billy Leonard said Mr Stewart had compromised his position as an elected representative.
“Dessie Stewart has admitted that he pretended to be someone else just to get some more votes,” he said.
“How can he continue to operate as an elected representative or go again at a future election and ask people for a vote?
“I hope that his reflection on his position is lot more thoughtful than his actions last May.
“It would be better if he went voluntarily rather than be forced out.”
SDLP assembly member John Dallat said Mr Stewart should resign immediately.
“This is a most serious case of electoral fraud,” he said.
“ We may never know the extent of it and the fact that Councillor Stewart only changed his plea at the last minute only adds to the seriousness of it.
“This must come as a huge embarrassment to the DUP and in particular to the Westminster MP for East Derry Gregory Campbell.”