Manchester United goalkeeper David de Gea has been warned Atletico Madrid fans will not forgive him if he joins cross-city rivals Real.

The 24-year-old has this week been seen in Madrid, increasing speculation he will leave Old Trafford for the Bernabeu.

He insisted he was there only to 'relax', and 'on vacation', but he is a target for Real Madrid, who rarely fail in their pursuit of players.

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Manchester United and Spain goalkeeper David de Gea arrives in Madrid earlier this week for a 'vacation'

De Gea posted this picture with his girlfriend Edurne Garcia to his official Twitter account on Thursday night

De Gea joined United from Atletico, where he started his career, for £18.9million in 2011. A year earlier he was a crucial figure as Atleti won the Europa League.

Tomas Ujfalusi was part of De Gea's defence that season and popular with the club's fans after spending three years in the Spanish capital; De Gea was equally liked, but Ujfalusi believes he will throw that away if he signs for Real because Atletico's fans would feel betrayed.

'No, no, no. They are not going to forgive him [if he joins Real Madrid], for sure,' the retired defender said.

'It's not going to be easy for him to make this decision.

De Gea (left) was a team-mate of Tomas Ujfalusi (back row, centre) during his time at Atletico Madrid

Ujfalusi believes Atletico Madrid's supporters will not forgive De Gea if he joins Real Madrid this summer

'It's also a surprise for me because he's an Atletico Madrid fan – when he was young his heart was with Atletico Madrid.'

De Gea initially struggled when he arrived in Manchester as the retired Edwin van der Sar's successor.

The pressure could be even greater at Real – where he is seen as the long-term replacement to Spanish icon Iker Casillas for both club and country – but Ujfalusi has complete confidence in his ability to succeed.

When the former Czech Republic international left Atletico in 2011, Thibaut Courtois was arriving on loan from Chelsea. He also won 78 caps playing alongside compatriot Petr Cech with the national team.

De Gea could be heading out of Old Trafford this summer but only if Real Madrid stump up £35m

The 24-year-old goalkeeper is keen on a move to Real Madrid, despite his links with city rivals Atletico

Few, therefore, are in a position to judge three of the world's best goalkeepers in such depth, but there is little doubt about who he believes is best.

'I have to say De Gea [is superior to Cech and Courtois],' Ujfalusi said.

'I was working with him for more than two years, and he was playing really well.

'It's difficult to say, but Petr Cech is 10 years older than De Gea and Courtois.