Real bosses are poised to table a £60m bid for the Spanish international in a move which is guaranteed to increase the bad blood between the two clubs.
United and Real are both in Los Angeles on their pre-season tours and sharing the same training base at UCLA.
The European giants have already fallen out over who has the best training facilities and hotels.
Now Starsport understands Real will infuriate United bosses even further with a multi-million pound bid that will be rejected out of hand.
United have made it clear De Gea is not for sale, but Real know he harbours a long-standing ambition to return to Madrid and will not give up hope of luring him to the Bernabeu.
The Champions League holders failed in a deadline day bid to land him in August 2015 when there was a mix up with the paperwork.
Real blamed United for the cock-up and criticised them in public.
De Gea, 26, went on to signed a new five-year contract worth £200,000-a-week that made him the highest-paid keeper in world football.
But he still has designs on move backing to the Spanish capital where he began his career with Real’s bitter-rivals Atletico Madrid.
De Gea is not agitating for a move but is understood to have asked his representatives what Real’s intentions are.
Meanwhile, the UCLA campus had to be evacuated on Wednesday night due to a bomb scare when a suspect package was found.
