Leeds United have denied release clause exists.
Cook, 19, is Leeds United's prize asset and has been playing regular Championship football for two seasons now.
Leeds United issued a statement on on their website yesterday denying the clause exists.
The Yorkshire Evening Post had reported Cook would be available at £5 million in the summer if Leeds, as expected, fail to clinch promotion.
This would have been disastrous for Leeds as it would have allowed their key midfielder to leave well below his market value.
Have Manchester United's chances gone?
A release clause might just have been the only way Manchester United could have got their hands on Lewis Cook this summer.
The bitter rivalry between the two clubs could mean Leeds could point blank refuse to do business with the Red Devils, despite selling them Eric Cantona, Rio Ferdinand and Alan Smith in the past.
Each circumstance was slightly different to this one, and Manchester United are no longer the 'must move to' club they were when Ferdinand and Smith signed.
Lewis Cook will have other options, and Leeds can to some extent control where he goes if they do sell him this summer for a higher fee.
Cook's contract expires in June 2017 meaning they need to tie him to a new deal or next January they will be in the same circumstances as they were with Sam Byram.
Even on a free transfer in 2017 if it came to it, Manchester United would also have to hope Leeds lad Cook would be open to risking fan anger by making the switch. For a teenager who will have other options, he might decide it is not worth the hassle especially at this stage of his career.
Seeing Cook in a Manchester United shirt would be a nightmare proposition for Leeds fans, but the prospect of this happening appears unlikely.