Newcastle owner Ashley holds an 8.92 per cent stake in the financially stricken Glasgow club and has previously pushed for complete control.
A proposal from Ashley's holding company MASH Holdings last month to increase his stake above 10 per cent was rejected by the Scottish Football Association (SFA), who have issued a complaint against Rangers and Ashley regarding breaches of dual interest rules.
Rangers and Ashley now face a disciplinary hearing later this month, while NUST have confirmed that they have drafted letters to the government's Secretary of State for sport, as well as the Premier League, the Football Association and the Football League to raise concern over the impact the sports merchandise magnate's interest could have on both clubs.
UEFA regulations prohibit any club involved in European competition from having an influence on "the management, administration and/or sporting performance of more than one club participating in a UEFA club competition".
Phoenix Suns owner Robert Sarver has seen a bid for a takeover of Rangers rejected and, with the Ibrox club now appointing Barry Leach - an executive at Ashley's sportswear chain Sports Direct - as their finance director, NUST are keen for their concerns to be brought to light.
"If Ashley's involvement with Rangers was to cease, then we wouldn't have an interest in writing to anyone as the circumstances would be removed," NUST board member Michael Martin told Perform.
"As long as he owns Newcastle and he has a position of great influence at Rangers, then obviously that means that either one of the clubs if we were to qualify for Europe at the same time, either one of us wouldn't be able to compete.
"You can expect Rangers, in the not too distant future, to get back to where a club of their stature should be. It's not beyond the realms of possibility of Newcastle United to be playing in Europe again.
"When he's got an interest in both clubs, it is an issue and it needs clarification. We're not saying that UEFA regulations definitely say one thing or the other, but they do raise a big, big question that needs to be clarified.
"And Newcastle, in their actions in banning a local journalist for raising the question, certainly seems to be very, very suspicious."
Martin also indicated that the issue lends credence to the widely held view that Ashley does not want European football at Newcastle, and expressed suspicions over caretaker boss John Carver's team selection in the 1-0 FA Cup third-round loss to Leicester City last weekend.
He added: "That is a widely held view among Newcastle United supporters that Ashley just wants Newcastle to be a mid-table, mediocre football club which doesn't compete for anything other than its status in the Premier League and the continuation of their revenue from television money.
"How come all these first-team players get little niggles at the same time every year, just when Newcastle are going to be playing an FA Cup year?
"And their record in FA Cup ties in recent years is absolutely abysmal over the last five, six years.
"It doesn't inspire confidence when there are players on the bench that never ever made a first-team appearance or even been in the first-team pool, then all of a sudden they're there on Saturday with 4,500 Newcastle supporters in the away end."
