• Money from prospective sale would fund up to 1,500 new pitches • Fulham owner Khan hopes to complete sale in two to three months

The Football Association has issued a guarantee that all proceeds of the pending £600m sale of Wembley to Shahid Khan will be used to fund grassroots football.

The FA chairman, Greg Clarke, sent a letter to members of the FA Council to reassure them the money would be funnelled back into the sport at participation level, including the purchase of an estimated 1,500 new pitches, improved drainage systems and floodlights. A source close to the FA told the Guardian a public commitment to investing such a massive sum of money in grassroots sport was “unprecedented and a potential game-changer for football in this country”.

Khan told the Guardian he hoped to have the sale completed in two to three months. The Fulham owner has been in serious discussions with the governing body since meeting the FA chief executive, Martin Glenn, at the Superbowl in the United States last year.

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As part of the negotiation and due diligence process the FA is seeking assurances from Khan, who also owns the NFL team Jacksonville Jaguars, about his long-term plans for the national stadium and particularly what would happen if he were to sell on in the future. Khan, who amassed a £5.2bn fortune from a car parts business, has been at pains to stress the stadium will remain the home of the England football team. The FA would retain the estimated £300m revenue from the Club Wembley hospitality and debenture business. England would be forced to go on the road for their autumn internationals between September and December but the FA Cup final, the Football League play-off finals and rugby league’s Challenge Cup final would remain at Wembley.

However, the FA is understandably concerned about the potential threat to those agreements if Khan were to sell on to a third party in the future. A source at the FA said: “Those of course are questions that need to be asked at this stage, the selling-on clauses are all part of the negotiation process.”

The governing body is concurrently holding talks with Sport England in an effort to persuade the board to release the £113m debt owed from the redevelopment of the stadium in the early part of the century. The Department for Digital, Culture, Media and Sport, Sport England and the Greater London Authority combined to contribute £161m to the construction of the £757m stadium in north-west London, which reopened in 2006. It is understood that debt cannot be wiped entirely but could be reassigned to developing football at grassroots level.

It is thought Sport England would be amenable to such a deal. Clarke wrote to the FA Council members to outline plans for a trust to manage the proceeds of the potential sale. “Should the board agree to proceed with a sale of Wembley, it is our intention that all proceeds would be placed in trust by the FA with governance approved by football stakeholders, government and Sport England to ensure they are allocated to closing the facilities gap and building community assets up and down England,” Clarke wrote. “The process to evaluate the feasibility of selling Wembley Stadium to fund our national facilities deficit has been developed thoroughly over several months by the executive with the help of a major city adviser.”

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Khan, the 217th richest person in the world according to the 2018 Forbes rich list, has stated he intends for only two Jaguars games to be held each season initially and that he considers the Wembley name to be sacrosanct. But the news of his bid was nevertheless viewed suspiciously in some quarters.

The former Chelsea and Leeds owner Ken Bates, a key figure in the stadium’s troubled redevelopment, said he felt the deal was being performed in an underhand manner. “The ground does not belong to the directors of the FA; they are custodians. It belongs to the English fans and their children, their grandchildren and great-grandchildren.”

Among current Premier League managers the reaction to the possible sale of the stadium was largely positive. Everton’s Sam Allardyce, who managed England for one match, welcomed the news providing the stadium could still be used for football. “The FA went through a hugely difficult period to build that stadium, so to lose it and not play there would be a no from me,” he said. “There would have to be a certain contract that makes sure Wembley is still Wembley and cup finals and some international games are still played there. But travelling across the country with England would balance that off very well. With the extra money coming into the FA – spent in the right way on development, grass roots and football in general – that would be a very good thing.”