The newsletter was sent to Labour Party members in the city
An election row has broken out in Brighton after the Labour Party implied it had seen some of the postal votes.
A newsletter was sent out by the party to its members saying the first postal ballots were being opened and checked.
It said during the council administrative process the party was able to "sample" how people were voting.
The letter was sent out by Brighton and Hove City Council leader Ken Bodfish.
The Labour Party insists it has done nothing wrong - and its view is backed by the council's chief executive and returning officer.
We are certain the secrecy of the ballot has not been compromised
But Keith Taylor from the Green Party said: "If what Ken Bodfish is telling his members is true, this breaks all the rules and makes nonsense of voter confidentiality.
"The Greens have been opposed to the all-postal ballot ever since New Labour came up with the idea.
"To us the idea is dangerous in terms of fraud and confidentiality."
Councillor Brian Oxley, leader of the Conservative group at the council, called Mr Bodfish's letter "irresponsible in the extreme".
He added: "His letter is badly worded and I have called on him to clarify what he means by 'sample' the votes, as election law requires the confidentiality of the ballots is maintained at all times."
But Labour argue they have not done anything wrong and were simply using published figures to give an idea of voter turnout.
The council's chief executive and returning officer, David Panter, confirmed the ballot had not been tampered with.
He said: "I want to assure the voters in the city that votes and ballot papers have not been tampered with in any way.
"What we have here is one party, the Labour Party, saying something inappropriately and not clearly to their workers, then another party, the Green Party, making mayhem with that."
People who are opposed to postal voting - namely the Green Party - have been attempting to make mischief out of this
Mr Bodfish defended his party's action and the postal ballot process.
"We have no idea whatsoever how an individual has voted or who that individual is," he said.
"All we are able to do is surmise that in Labour wards the turnout was not as high as in Conservative wards.
"People who are opposed to postal voting - namely the Green Party - have been attempting to make mischief out of this."
In a statement the Labour Party said it supported unreservedly the secrecy of the ballot.
"The turnout figures and the register of who has voted in the postal ballot in Brighton are published on a regular basis by the returning officer," the party said.
"The use of such turnout figures by any political party is perfectly proper.
"We are certain the secrecy of the ballot has not been compromised."