Blogger Andrew Loh has apologised to President Tony Tan Keng Yam for an expletive-laden Facebook post directed at him.
Late Wednesday night, Mr Loh issued a statement on his own Facebook page, saying he offered his "sincerest apology to the President and to Singaporeans" and admitted that he had used "harsh and insulting" words.
On Sunday, Mr Loh had reposted a Facebook update by President Tan about the haze, adding his own critical comments.
But in criticising the President for what he thought was a lack of action on the haze, he used vulgarities multiple times.
Mr Loh said on Wednesday he would be apologising to President Tan in writing.
There was no indication that Dr Tan himself had asked for an apology or took any other action against Mr Loh.
But some netizens have since latched on to Mr Loh's postings and criticised him for insulting the President.
Wrote netizen Dawn Goh on Facebook: "There is never a reason to use such foul language on anyone. Certainly not for our Head of State."
Another, who went by the name Qiu Yung, said: "I don't think one should excuse rude behaviour by saying that respect or true respect has to be earned.
"We always begin by respecting people even if you don't like them."
Mr Loh on Wednesday night acknowledged he was in the wrong: "The words I used were harsh and insulting, and the sentiments I expressed about the President unfair and untrue.
"I also apologise to Singaporeans for insulting the President and the office that he holds. I would like to thank my fellow netizens for pointing out my failings. I will keep this lesson in mind," he added.
Mr Loh is one of the founders of sociopolitical site The Online Citizen.
He has since moved on to start his own website and also writes for Yahoo Singapore.
There had been isolated calls online on Wednesday for complaints to be lodged with Yahoo.