The musical theatre competition has been locked in a ratings battle with The Voice UK since it began in January.
Let It Shine won more viewers than its ITV rival on its debut but, since the second episode, The Voice has taken a narrow lead.
Gary, 46, reportedly wants to give his show the edge by bringing in his old bandmates.
But unfortunately for fans of the original fivesome, one member of the band will not be in attendance.
A source told The Sun: “Gary wanted to finish off Let It Shine with a bang and he thought a Take That reunion to close the show was the perfect way to achieve it.
Let It Shine: Gary Barlow is planning a Take That reunion for the final episode
Let It Shine: Jason Orange (pictured far left) will not be in attendance
Gary decided not to even ask Jason
“The idea is that it would also set the stage nicely for the musical to follow.
“Robbie [Williams] seems very keen to get on board, so things are looking good. But in the end, Gary decided not to even ask Jason [Orange].”
Robbie’s inclusion is a surprising one as the singer only reunited briefly with his bandmates in 2010, having quit the group the first time in 1995.
In 2014, Jason left Take That and Gary, Howard Donald and Mark Owen continued as a three piece.
Let It Shine: Robbie Williams is said to be keen to appear on the show
Theatre talent shows through the years
Wed, November 16, 2016
As Gary Barlow launches new talent show 'Let it Shine' to find the stars of a Take That musical, we look at the successful musical theatre talent shows of the past.
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Let it Shine – The new BBC show will see Gary Barlow looking for five young men to star in a new Take That musical
Gary is said to have felt that asking Jason back was pointless as he had made it clear he was no longer interested.
The insider continued: “With him out of the picture, Gary is pouring his energy into making sure their reunion as a four-piece is a huge success and blows the audience away."
Express.co.uk has contacted a representative for Gary Barlow asking for comment.
Let It Shine continues next Saturday at 6.55pm on BBC One.