A Scottish daily has found itself at the centre of a political row after it was revealed that a columnist had received an undisclosed payment from a pro-independence campaign group.

The Herald published an article by academic Dr Elliot Bulmer last month and it has now emerged that he received an payment of around £100 from the Yes Scotland campaign for the piece, which was about a proposed Scottish constitution.

Officials who are against Scottish independence are now planning to complain to the Scottish charities regulator because Dr Bulmer is a research director for the Constitutional Commission, which describes itself as a politically neutral charity.

Hacking claims have also been made by Yes Scotland because it believes private emails were accessed to gain information relating to the fee and police are now investigating this.

In a story about the incident in The Herald, a spokesman for the Herald & Times Group said: “We were offered an article for our opinion pages by a constitutional expert, which we accepted in good faith as a valid contribution to the referendum debate.

“We declined a request for payment and the author’s background was made clear to our readers.”

Yes Scotland has said it had no influence over what Dr Bulmer wrote and he was paid a “nominal fee” for time and effort he spent on it.

A spokesman said: “This matter was first brought to our attention last Wednesday when we were asked for comment on Dr Bulmer and the article in question. We responded quickly, confirming that a small fee had been paid to Dr Bulmer at his request. We were perfectly relaxed and transparent about this.

“However, later that day it became apparent that an email account at Yes Scotland had been accessed illegally and that the information relating to this matter had been gleaned as a result.

“We alerted the police and British Telecom as well as the enquirer who, upon reflection, decided to not proceed further.

“Given that the illegal breach of Yes Scotland email has become the subject of an extensive and ongoing police inquiry involving detectives from Police Scotland’s Digital Forensics Unit, we have – under legal advice and at the request of the investigating officers – been unable to discuss the content of the email relating to Dr Bulmer.

“However, given persistent unhelpful speculation, we can confirm that in the course of a wide-ranging discussion with Dr Bulmer it was suggested that he, as an academic working in a private capacity, might consider writing an article on matters about constitutional frameworks based on his expertise.

“At his request, he was paid a nominal fee for the considerable time and effort he spent on it. We had no input to, or any influence over, what he wrote.”

Blair McDougall, director of the Better Together campaign, said: “These allegations against Yes Scotland, if true, fatally undermine trust in the independence campaign. How can we believe what they say if they are secretly paying off supposedly impartial experts?

“The leadership of Yes Scotland must take responsibility for this and answer these accusations.”