The diplomatically conducted correspondence sheds a remarkable light on the delicate constitutional relations between the Queen and her ministers.
After the Queen raised the issue with Harold Macmillan at a private audience, he consulted Peter Thorneycroft, the Minister of Defence, who explained in a confidential minute that the Queen's flights were subject to special air traffic control arrangements known as Purple Airways.
The request for advice from the minister followed a private minute to Macmillan from Sir Burke Trend, the Cabinet Secretary who told the Prime Minister that the matter was too sensitive to be discussed by the full Cabinet. Mr Thorneycroft explained that Purple Airways meant that other air traffic on the same routes was 'kept widely separate from the royal aircraft' and the 'risk of mishap is therefore small'. Mr Thorneycroft recommended that there should be no objection to her travelling with 'one of Her sons'. But he added: 'Conversely I think that the precaution by which She would never travel with more than one of Her sons should apply to all forms of transport.'
As a result, Macmillan wrote to the Queen saying that while 'ministers would agree' to her travelling with one of her sons on internal flights, they would feel 'more anxiety' if she travelled with both Prince Charles and Prince Andrew on the same flight.
On 21 May, the corespondence closed with a letter to the Captain of the Queen's Flight from Sir Michael Adeane, the Queen's Private Secretary, saying that she would, if necessary, henceforth travel within the UK with 'either the Prince of the Wales or Prince Andrew'.
The letter adds: 'In either set of circumstances Princess Anne, if necessary, could be added to the party.'
The letter concludes: 'In reaching this decision the Queen has had the advice of her Government who have agreed to this current change in procedure.'