Manchester United have been alerted to Gareth Bale’s potential availability in the summer - but Real Madrid will want David de Gea to be involved in any deal to take the world’s most expensive player to Old Trafford.

Louis van Gaal’s hopes of landing the Welshman will also be dependent on United securing Champions League football next season.

Sources in Spain have indicated the European champions are willing to let Bale leave the Bernabeu at the end of the current campaign - placing the Premier League’s top clubs on standby.

Chelsea are also monitoring his situation, while City are expected to heavily reinvest in their ageing squad next summer.

But United hold the trump card, with De Gea one of Real’s top targets.

The Spanish giants are preparing a move for the 24-year-old goalkeeper regardless of Bale’s situation.

They see him as the man to succeed Iker Casillas for club and country.

United will know that the prospect of returning to his homeland will be tempting for De Gea sooner or later and must decide whether to capitalise on Real’s interest by using him as a bargaining chip for Bale.

De Gea has established himself as one of Europe’s top keepers after overcoming a disastrous start to life at Old Trafford.

Last season he was named United’s double player of the year and he has been in outstanding form again this term.

But just as with Cristiano Ronaldo, United will be pragmatic about the chances of keeping him in England for the rest of his career.

And, like with Ronaldo - who joined Real for a world record £80m in 2009 - they can expect to make a significant profit on the £18.9m Sir Alex Ferguson paid for him in 2011.

They could expect to demand a world record fee for a goalkeeper, in excess of the £32.6mm Juventus spent on Gianluigi Buffon in 2001.

With Real determined to recoup much of Bale’s £85.3m fee, De Gea would prove a sizeable makeweight.

And with former Barcelona No.1 Victor Valdes currently training with United as he recovers from injury, they could have a ready-made replacement waiting in the wings.

Real’s willingness to cash in on Bale comes as a surprise, given his impressive first season in Spain where he played a pivotal part in their Champions League and Spanish Cup success - scoring in the finals of both competitions.

But the 25-year-old still isn’t considered among the absolute elite along with Ronaldo, Lionel Messi, Neymar and Luis Suarez.

Real are not making his potential availability public, but United are among the clubs who are aware of his situation.

They tried to pip Real to his signature when he left Tottenham for La Liga two summers ago.

He is precisely the type of superstar name executive vice chairman Ed Woodward believes United should always be competing for.

Despite spending £150m in summer - including the British record £59.7m signing of Angel di Maria - Woodward is prepared to make more cash available to van Gaal.

A £44m deal has been put in place to make Radamel Falcao’s loan move from Monaco permanent and Roma’s Kevin Strootman is another target.

But with concerns over 31-year-old Robin van Persie’s form, United will explore the possibility of adding more attacking talent.

The sort of figures required to sign Bale will be dependent on the promise of Champions League revenue.

A top four finish is the minimum requirement for van Gaal, with the Dutchman made aware that United’s investment in the summer was made with a view to an immediate return to Europe’s premier competition.

Bale’s willingness to join a club that is unable to offer him Champions League football would also be in doubt.

Tottenham’s failure to qualify for the competition was a factor in his decision to leave White Hart Lane. Having triumphed in Europe in his first season in Spain, he will be determined to continue competing at the highest level.