The SDLP MLA Claire Hanna has tweeted a photo of her daughter appearing to mark her referendum ballot paper in a polling station booth.
It was posted on Thursday on Twitter by the south Belfast politician.
A number of politicians from Northern Ireland tweeted pictures of their ballot papers.
However, Ms Hanna's photo showed her daughter holding a pencil with her hand, appearing to mark a ballot paper for the EU Referendum.
A spokesperson for the Electoral Office said: "We would strongly discourage anyone from taking photographs inside the polling station."
The spokesperson added that "the requirement for secrecy around the conduct of the ballot is set out in the Representation of the People Act (1983)".
The BBC contacted the SDLP press office to get a comment from Claire Hanna but is still awaiting a reply.
However, in a Facebook post, Ms Hanna said: "This is literally the stupidest article I have ever read on what is the absolute opposite of a slow news day."
Northern Ireland has voted to remain in the EU Referendum by a majority of 56% to 44%.
However, the UK, as a whole, has voted by a narrow margin to leave the EU.
Overall, 440,437 people in Northern Ireland voted to Remain in the EU and 349,442 to Leave. Out of 18 constituencies, 11 voted to stay in the European Union.
The turnout in Northern Ireland was 62.7% with 789,879 people voting in the referendum.