Mohammed Mehmet has asked for pressure over the count to stop
Political parties have put aside their rivalries to demand all votes in the assembly election are counted overnight.
Leaders of the four main parties say a plan to start counting votes in north Wales the day after the election could cause an "unacceptable" delay.
Counting elsewhere will start when polls close at 2200 BST on 5 May.
But counting officers in the north said arrangements were in place and they did not plan to change their minds.
In a letter to the region's chief counting officer, Dr Mohammed Mehmet, the leaders of Welsh Labour, the Welsh Liberal Democrats, the Welsh Conservatives and Plaid Cymru say it is "wrong that the people of north Wales should be treated differently".
They say a stipulation by the Electoral Commission that votes in the AV referendum - being held the same day - should not be counted until 1600 BST on Friday 6 May means if there are re-counts "the entire process could quickly fall into chaos and confusion".
They say hold-ups to declarations in north Wales would mean media coverage of the election could be overshadowed by results from Scotland's devolved elections, English local elections and the AV referendum.
Delaying the north Wales count could delay the overall result of the Welsh election, the leaders say.
"We believe that such a delay would be unacceptable to the people of Wales as a whole - not just to the people of north Wales," they add.
Counting for the 13 seats in north Wales is scheduled to start at 0900 BST on 6 May.
Marginal seats in the region could play a big part in deciding whether any one party wins a majority.
'Enormously frustrating'
Denbighshire council, where Dr Mehmet is chief executive, confirmed it had received the letter.
In a statement on behalf of the north Wales constituencies, it said: "Arrangements are already in place to count election votes on the Friday and there are no plans to change those arrangements."
At the weekend in an e-mail responding to criticism, Dr Mehmet pleaded with politicians to stop their pressure.
"Some of us can deal with it, but it is beginning to unsettle more junior staff and we, as returning officers, must look after their health and well being," he wrote.
He said returning officers considered counting overnight, but decided to stick by their decision.
They had done an enormous amount of planning for the count and it was "simply too risky to change our minds at this very late stage", Dr Mehment said in the email.
He added: "I have to say that it is also enormously frustrating that other areas have taken their decisions so late and that the political expressions of concerns have been so last minute."
