Lamb has flatly rejected the story and said the 20-year-old stopper is not for sale at any price.
Lamb insisted: "The story is complete nonsense. We have no intention of selling Jason for £4million or any other fee.
"He is a massive part of the future of this football club, so the idea of selling him is not on our agenda."
Boro boss Tony Mowbray added: "I think it's vitally important that we don't sell our assets, of which Jason is certainly one.
"We don't need to sell our young players. They can get us back in the Premier League if we give them time to fulfil their potential here."
Source: PA
Source: PA