The full back-page advert in the Scottish Catholic Observer portrayed a photo of Lord Reid alongside his personal plea for readers to vote No in September's referendum, but it is alleged that he broke electoral law by failing to specify who had promoted or paid for the advert.
Sarah Mackie, of the Electoral Commission, said: "I can confirm that we have received a complaint regarding an alleged breach of the referendum rules on imprints.
"We are now considering the complaint and are unable to comment any further while the matter remains under consideration."
Lord Reid is pictured alongside quotes urging readers: "I am a proud Scot and want the best for our country. That's why I will be voting No to separation in September.
"A No vote is a vote to protect the best interests of Scotland and its people. Being part of the UK means we can have the best of both worlds for Scotland.
"We can have our own unique identity here in Scotland and benefit from the security of something that comes from being part of something bigger. Only separation puts that at risk."
He signed the statement: John Reid, former Labour Cabinet Minister and former chairman of Celtic FC.
The problem for Lord Reid is that a political advertisement appeared without the imprint required under electoral law, stating who is promoting and paying for it. Also, if he is paying himself for a series of adverts over the next three months it may trigger the outlay requiring him to register individually as a permitted participant in the process.
The Better Together Campaign indicated that it had no part in placing the adverts, with sources pointing to an individual donor.