Manchester United are set for a protracted summer battle with Real Madrid over the transfer of David de Gea after it emerged they plan to open the bidding with an offer as low as £13million.

De Gea seems destined for the Bernabeu after telling team-mates and staff at Old Trafford that he is leaving the club and failing to respond to an offer of a £200,000-a-week new contract.

The United goalkeeper still remains concerned about the presence of Madrid icon Iker Casillas at the Spanish club and his representatives are still waiting for assurances from president Florentino Perez that De Gea will be an automatic pick as No 1 next season.

Real Madrid are looking for a buyer for Iker Casillas (left) as they hope to sign Man United No 1 David de Gea

However, Casillas (third left) was part of a star-studded Real line-up to promote their new kit for next season

The Manchester United keeper has been strongly linked with a summer move to Real Madrid

De Gea arrived at Old Trafford in 2011 from La Liga outfit Atletico Madrid and has become a fans' favourite

That is not expected to delay things too much, however, unlike Real’s intention to start the bidding low.

United have a valuation of £25m and above for De Gea and will expect Real to match it.

However, the Spanish club will argue that De Gea only has a year left on his current contract in England and, therefore, should be sold for much less than his actual ‘value’.

Red Devils boss Louis van Gaal does not want to lose De Gea but a move to Real looks on the cards

United see Hugo Lloris as the replacement for De Gea but doubt whether they can get a deal over the line

Lloris, pictured as France are beaten 4-3 by Belgium at the weekend, wants Champions League football

Ajax keeper Jasper Cillessen is also viewed as an attractive proposition by United this summer

With United keen to show their supporters that they will not be messed about by one of their traditional European rivals, this could all add up to a long and tortuous process for fans who just want to put the whole business behind them and move forwards with a new goalkeeper.

In terms of a replacement for De Gea, United have a strong interest in Dutch No 1 Jasper Cillessen – as revealed by Sportsmail last month – and Hugo Lloris at Tottenham.