Zadkovich has been given time off since his involvement with the Socceroos at the East Asian Cup in South Korea last month, where he played two full games, and the Jets had approved the midfielder's trip to the United Kingdom to visit family.
 
Newcastle CEO Robbie Middleby confirmed Zadkovich had 'accepted an invitation to visit' Millwall but it appears reports of a trial were overstated, while the Jets have yet to hear anything official from the English club.
 
"Ruben was scheduled to have a week off following his commitments with the Socceroos," Middleby said in a written statement released by the Jets.
 
"He asked for the opportunity to take up Millwall's offer of a visit during this time and we have accommodated that request.
 
"At this stage, we have not received any formal offer for Ruben and he remains a Newcastle Jets player."
 
Socceroos coach Holger Osieck's decision to start Zadkovich in Australia's opening two matches in South Korea surprised many with the feisty midfielder rarely mentioned as a possible international player.
 
But Zadkovich's performances for Australia have caught the eye of new Millwall manager Steve Lomas, who has two other Australians on his books: young left-back Shane Lowry and former Middlesbrough striker Scott McDonald.
 
If Millwall want to sign Zadkovich, it seems they will have to pay a transfer fee, as the Jets made it clear their captain, who is contracted for the next two A-League seasons, is vitally important to Gary Van Egmond's squad.
 
"Ruben is our captain and a highly valued member of our squad and our preference is for him to stay at the club for the remainder of his contract," Middleby said.
 
"We're looking forward to having him back in Newcastle for pre-season training this week."
 
Zadkovich played 26 matches last season for Newcastle and took over the captaincy after former skipper Jobe Wheelhouse quit in February.
 
Meanwhile, Newcastle's marquee striker Emile Heskey is expected to arrive back in Australia this week to begin pre-season training for his second A-League campaign.
 
Heskey scored nine goals and set up another two in his 23 appearances for Newcastle in the 2012/13 season.