Gill acknowledged that the family may be forced to appease growing supporter resentment by taking a higher profile, but said the Knights, led by Goldman Sachs's chief economist and former United board member Jim O'Neil, were wasting their time.
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'That's not to say that people like the Red Knights won't come and think that they can put a plan to them. But unless the owners want to sell, which they've given no indication to me at all that that is the case, then they can't buy the asset. It's not for sale.'
Gill said that he has not heard directly from O'Neil, who is a friend as well as a former colleague, or fellow Knight Mark Robinson, a partner at City law firm Freshfields who advised the club board when it was resisting the Glazers in 2004.
He did not criticise them directly, but was withering about Harris, the executive chairman of executive bank Seymour Pearce who has been involved in several high-profile Premier League takeovers including Roman Abramovich's purchase of Chelsea.
'Keith Harris will go anywhere that there's a bit of publicity around and we know that and we accept that,' Gill said. 'That's his modus operandi, but his track record in football isn't anything to write home about. That's my view of that, but these are credible people and they do what they think is in the best interests of the club but it's not going to take them anywhere if the owners, the Glazers, have no wish to sell. From our perspective, they are running the club in the right way.'
Gill said that the club's financial situation following the £507 million bond issue was 'entirely appropriate', with annual interest payments of around £45 million covered by the club's growing revenues.
By contrast he questioned how the Red Knights proposed to run the club given their proposal to raise funds from up to 40 wealthy individuals.
'The Red Knights' idea of having 20, 30 or 40 very wealthy people owning and running Manchester United, I just don't know how it would work in practice,' Gill said.
'The best clubs, the better run clubs have clear single-decision making that iws quick and efficient. I don't see how if you've got a number of very wealthy people being involved [that can happen].
'They don't become wealthy through luck, those sort of people want to be involved in the decision making, The key clubs, Abramovich at Chelsea, Mansour at Man City, Berlusconi in Milan, even the president, the key decision maker at Madrid is not all those fans, it's the president.
'I'm not sure what the end game is but the end game is irrelevant because the owners are long-term investors and want to keep the club for many years to come.'
The Red Knights are hoping to harness supporter unrest at the club's debts of £709 million, and Gill acknowledged that the Glazers may have to take a more visible role to appease fans who turned Wembley green-and-yellow at Sunday's Carling Cup final.
'What they [the Glazers] done is let me and my team run the business, they've let Alex run the football side, and if you look at our financial results and our on-field performance, I think that's vindicated that approach. If, in the future, in order to appease people or to explain the long-term nature of their investment, that [a higher profile] may be necessary.'
Gill's assault came as the Red Knights were assessing the impact of their public declaration of intent. A spokesman for the group said that they had received 'an international response' from potential investors, and the Manchester United Supporters Trust reported that its membership had doubled overnight to 78,000, larger than the capacity at Old Trafford.