The Monarch, along with the Duke and Duchess of Cambridge and Prince Harry are due to sit down with 10,000 guests at the Patron’s Lunch next month.

A mystery millionaire has spared the blushes of the Queen’s grandson Peter Phillips by buying £100,000-worth of tickets for her 90th birthday party on The Mall.

With charities struggling to afford the £150-a-head asking price for the street party tickets, it had been feared that the highlight of Her Majesty’s birthday celebrations would be a flop.

But a benefactor has stepped in at the 11th hour – on condition of anonymity – and bought hundreds of unsold seats for charities, the Mail can reveal.

The Monarch, along with the Duke and Duchess of Cambridge and Prince Harry are due to sit down with 10,000 guests at the Patron’s Lunch next month.

Yet even with the anonymous donor’s help, a quarter of the tickets have had to be sold to the public, suggesting the situation was more dire than first thought.

Mr Phillips, who is the London director of Australian firm Sports and Entertainment Limited, initially announced just 1,000 of the 10,000 tickets would be available to the public, with the rest going to the Queen’s charities.

That figure has now risen to 2,500, meaning at least a quarter of those who go to the event on June 12 will have no link to her charities at all.

Mr Phillips’ firm will take an undisclosed fee for the event as invited organisations struggle to find the cash for tickets. Charities have even been told they can auction up to 40 per cent of their allocation in order to raise money for their staff to be able to attend.

The Patron’s Lunch will be the highlight of three days of celebrations, which include a national service of thanksgiving at St Paul’s Cathedral and Trooping the Colour.

Guests at the street party – one of the biggest ever – will have a luxury hamper-style lunch provided by M&S and ‘carnival-style’ entertainment.

There has been criticism over Mr Phillips, who is Princess Anne’s son, being paid an undisclosed fee to organise the not-for-profit event.

His firm’s cut will be paid out of money raised by ticket sales and programmes. Any profit will go to the Queen’s charities.

Questions were raised over why organisations supported by the Monarch should have to pay to attend an official royal celebration of their role.

One charity worker described it as a ‘sting in the tail’.

It is thanks only to the anonymous donor that many smaller organisations are able to attend. A source said: ‘A donor approached Buckingham Palace saying they wanted to donate a number of tickets anonymously.

There has been criticism over Mr Phillips (pictured), who is Princess Anne’s son, being paid an undisclosed fee to organise the not-for-profit event.

‘The Palace approached the Patron’s Lunch organisers to see if they could help facilitate this and then wrote to the 600-plus organisations and charities involved in the event to let them know about the generous offer and the tickets were allocated.’

It has not been revealed how many tickets were donated but it is known to be up to a tenth of the 7,500 seats for charities.

A Patron’s Lunch spokesman said: ‘We are delighted that a number of tickets have been provided to patron’s organisations by a third-party donor.’

They highlighted that 76 per of the international organisations represented by the Queen and 90 per cent of UK-based charities were attending the event, which will be televised by the BBC, but it is unclear how many owe their attendance to the donation.

Mr Phillips has insisted he did not secure the contract to organise the event because the Queen is his grandmother.

But he has admitted that he had discussed it with her.