It is understood that the PSNI launched the probe after recordings were received in the post by Masonic officials at the headquarters of the Provincial Grand Lodge of Antrim at Rosemary Street in Belfast.

Police are investigating if the recordings are of secret rituals held in the building which is the main meeting place of freemasons in Co Antrim, one of the largest lodges in Ireland.

Sources have told the Telegraph that a CD containing a recording was recently sent to lodge officials at Rosemary Street.

Officials have been left mystified by the suspicions of secret recordings. There have been no demands or threats received by freemasons, but the police have been alerted.

A PSNI spokeswoman told the Belfast Telegraph: "A letter has been received and an investigation is ongoing."

A Masonic spokesman refused to comment on the allegations, but confirmed that police were carrying out an investigation.

He said: "I can confirm that a matter relating to the Provincial Masonic Hall, 15 Rosemary Street, Belfast, has been referred to the PSNI for investigation. At present we have nothing further to add."

Despite the probe the hall at Rosemary Street is still being used for meetings, including one as recently as last weekend.

However, a source told this paper: "This has been the talk of meetings all over Co Antrim. We have been told that something was received in the post which included a recording of some sort. Police have been asked to check out how this could have happened, if it is possible that the hall was bugged in some way.

"Everybody is mystified why anyone would want to do this, we are a charitable organisation."