Camilla Parker Bowles to appear in The Crown series three

"Peter's already talking about the most wonderful things," Mackie told the audience at the BFI in London where she appeared alongside Queen Elizabeth actress Claire Foy and director Philip Martin.

"You start meeting Camilla Parker Bowles in season three," she added. "We have to be honest, season three and four are being mapped out."

Camilla and Charles first met in the early 1970s, but Camilla went on to marry her first husband Andrew Parker Bowles in July 1973. Prince Charles married Princess Diana on 29 July 1981.

However, Camilla and Andrew divorced in 1995; later that year Princess Diana referred to Camilla during her infamous interview with Martin Bashir, saying, "There were three of us in this marriage."

On 9 April 2005, almost 30 years after they first met, Charles and Camilla were married in a civil ceremony, with Camilla taking the title HRH Duchess of Cornwall.

Martin and Mackie were tight-lipped on details about season two, but they did reveal where the next instalment will begin and end: in 1956 with the Suez Crisis, and in 1963 with the resignation of Prime Minister Howard Macmillan.

"It has a different flavour," producer Mackie said. "It feels like the 60s are with us and it has a slight shock of the new."

Director Martin added: "The first season happened in a bubble; I think that Elizabeth and Phillip and Margaret are all in a world and everybody is in some ways supportive of them.

"And I think in the second season the world comes crashing in."

The Crown producer Suzanne Mackie was speaking at the BFI & Radio Times Television Festival.