The Real Madrid star is apparently unhappy with life at the Bernabeu because he feels he deserves a contract equal to rival Lionel Messi’s at Barcelona.

Los Blancos’ shocking form, where they sit fourth in the La Liga table, has also added to his unrest.

And Jose Mourinho is said to be keen on being reunited with his former attacker at Old Trafford.

United are already expected to complete the signing of Alexis Sanchez from Arsenal in the coming days.

But with Mourinho set to sign a contract extension in Manchester he could be backed further with a record-breaking transfer.

Reports coming out of Spain suggest Man Utd are willing to smash the world transfer fee to bring the Portugal international back to where his career kicked into gear.

Journalist Jose Luis Sanchez told El Chiringuito TV that Man Utd are willing to invest €400milion (£352m) in the deal.

That fee is just under double the amount Paris Saint-Germain paid Barcelona for Neymar in the summer - the current world record.

However, Sanchez believes that Real should do all in their power to keep Ronaldo in the Spanish capital.

Sanchez said: “United are ready to invest to sign Cristiano for €400million and they have put the machinery on march.

“Madrid has to raise the funds to keep him. They cannot lose out.”

Fellow journalist Tomas Ranchero added: “If there is money in the box then you have to use it to keep the best player in the world.”

Ronaldo has been regularly linked with moving back to united ever since he left in 2009.

But as he reaches the tail-end of his career, the 32-year-old is looking ever the more likely to move onto pastures new.