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Ajax boss Frank de Boer has warned Manchester United that the asking price for keeper Jasper Cillessen is going up by the week.

As we revealed in May, United are plotting a £25million swoop for Cillessen if their Spanish No 1 David De Gea gets his wish to join Real Madrid.

United have told Madrid they want £35m for De Gea - resulting in a stand-off that has left Cillessen and his Dutch club in limbo.

And De Boer, who has missed out on three replacements for the Holland international, said: "Man United will have to pay huge price for Cillessen.

"Let it be clear that we are going to take a really strong attitude and we will fight against him leaving at this stage.

(Image: Getty)

"Why? Because we have seen all our three alternatives go out of sight and it gets harder and harder to find a decent replacement.

"To make it clear, Jasper is getting more expensive by the week, although there are certain figures that we can't turn down."

Ajax wanted Fulham's Maarten Stekelenburg, but saw the Dutchman join Southampton on loan.

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Then Arsenal's Wojciech Szczesny moved to Roma, while Michel Vorm is now likely to stay at Tottenham.

United boss Louis van Gaal gave his close friend De Boer an early warning last summer that he would be bidding for Daley Blind - and the Ajax boss appreciated the gesture.

De Boer added: "This time, we don't know what is happening.

(Image: Dean Mouhtaropoulos)

"This far in the window, I hope we hear nothing anymore from Manchester United."

De Gea's position at Old Trafford seemed to have become untenable when Van Gaal axed him from United's season-opening win over Tottenham.

Argentine stand-in Sergio Romero excelled with a last-minute save to thwart Christian Eriksen.

And although Van Gaal admitted afterwards he would continue to leave out De Gea while Madrid are circling, he did open the door for the Spaniard to return to his team once the transfer window has closed.

Van Gaal said: "A goalkeeper is the position where you need maximum concentration.

"You need the highest focus over 90 minutes and when you are in circumstances like David De Gea it is difficult to cope with that.

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"We have observed him in preparation and in matches and it is not the same De Gea as last year.

"But, after the situation has been cleared by September 1, we will look at it again."