The Hillsborough charity single 'He Ain't Heavy, He's My Brother' is now 3,400 copies ahead of the X Factor champion's winner's single 'Impossible'.
This comes just 24 hours after Arthur was leading the chart by 3,000 copies in the first midweek update yesterday (December 18).
"This really is the closest No.1 battle we can remember - even closer than the legendary 2009 battle between Rage Against the Machine and Joe McElderry," said Official Charts Company Managing Director Martin Talbot.
"The poll concludes at midnight on Saturday night [December 22], so there is a long way to go yet - but this one looks like going down to the wire."
Arthur has currently shifted 106,300 copies of 'Impossible' this week, while The Justice Collective are ahead with sales of 109,700.
The Justice Collective are made up of a group of musicians including Paul McCartney, Robbie Williams, Rebecca Ferguson and Melanie C.
Watch James Arthur chat to Digital Spy below: