Smarting Linfield have queried a Glentoran player's eligibility as the Blues look to hit back in an escalating tit-for-tat feud between Belfast's Big Two.
Last week, Sunday Life Sport revealed how the League champions were fined by the Irish FA for 'failing to control their supporters' during a recent derby match after Glentoran reported their rivals to the governing body.
Now, the Blues have questioned the registration of Glens player Cameron Stewart with the IFA.
It is understood Stewart - a former title winner with Linfield - was registered twice as a Glentoran player and Linfield's query is about the detail of the second registration.
The 23-year-old played in last month's Big Two derby, which ended 3-3 at Windsor Park.
"We received correspondence late on Friday night from the Irish FA on a minor technical issue and we are investigating it," said Glentoran Chairman Stephen Henderson.
An Irish FA spokesperson told Sunday Life Sport last night: "I can now confirm we have received correspondence on this matter and it will be looked into."
Sunday Life Sport has learned that Linfield sent correspondence to both the Northern Ireland Football League and the Irish FA asking for clarification on the eligibility status of Stewart in the last week.
The matter is now being addressed by an IFA registration sub-committee, who will deliberate on the issue before giving NIFL the result of their findings, after which Glentoran could face penalties.
Should the player be found to be technically ineligible, NIFL will review the games he featured in for Glentoran and may decide to issue fines or points deductions.
The move follows on from Sunday Life exclusively detailing the findings of an IFA disciplinary investigation into alleged sectarian singing from a section of the Linfield support at a recent Big Two derby at Windsor Park.
Two charges were found to be 'not proven' - including the charge of sectarian singing - but one charge of failing to control spectators was upheld and the Blues were fined £500.
However, Linfield chiefs were left dismayed at being reported by Glentoran, as they felt this was a departure from normal etiquette, and this move will clearly be viewed as retaliation by the Glens.
When approached by Sunday Life Sport yesterday, Linfield remained tight-lipped, issuing an emphatic "absolutely no comment" response to our enquiry.
NIFL also made no comment, citing the IFA's lead and jurisdiction on the matter.
But it is clear that Big Two relations are at an all-time low following a series of incidents in recent years, beginning with alleged abuse aimed at Glentoran owner Ali Pour and Mick McDermott's wife, Karla, last season at Windsor Park.
Glentoran were due to play Linfield at Windsor again on Boxing Day before the game was postponed due to Covid-19 restrictions, and Sunday Life Sport understands the Blues were prepared to offer only five directors' box tickets to the Glens, the bare minimum under NIFL regulations.
Meanwhile, following the postponements of the Carrick v Crusaders and Portadown v Larne games this weekend for Covid-19-related issues, the Crues yesterday announced four of their players have tested positive for coronavirus. The club are following NIFL's Covid-19 protocols.
Crusaders hope to be back in action on Saturday, January 2 when the Irish Premiership is set to resume with a full set of fixtures.
The League will be shut down over the festive period to comply with new Covid-19 regulations imposed by the Northern Ireland Executive, forcing the Boxing Day matches and those games on Tuesday, December 29 to be postponed.
However, it is understood club training will still be allowed during that week.