The Queen got into the festive spirit by helping to light a Christmas pudding during a lunch for volunteers and joked – “hold on to your hair”.

Camilla poured liberal amounts of brandy over the huge festive treat, the centrepiece of a Christmas lunch staged by the Royal Voluntary Service (RVS), before it was lit by the organisation’s chief executive Catherine Johnstone.

The King’s consort has been a long-term supporter of the RVS, working as a volunteer in previous years. She is now the organisation’s patron and she was joined by celebrity ambassadors Elaine Paige and Felicity Kendall at the event.

In an impromptu speech after the 10kg pudding burned with a blue flame, the Queen said she wanted “to thank you all, for all you do for the community”.

She added: “As I always say, this country would collapse without you so we’re eternally grateful for everything you do.”

The event, staged in Little Venice, central London, marked the start of the RVS’s “Festive Spread” campaign which will see volunteers hold more than 70 Christmas lunches across the country for people who may be spending the holiday period alone.

When Camilla arrived, Paige asked how she was and the royal, who in recent weeks has been experiencing the after-effects of a bout of pneumonia, replied “just standing”.

The Queen joined a number of tables where RVS volunteers were enjoying canapes before their meal began and she chatted to the organisation’s chairman Patrick Dunne, who was sitting with helpers.

Mr Dunne said afterwards: “She’s very at ease, she gets what it’s like to be a volunteer because she’s been a volunteer.”

Kendall, famed for her part in the 70s sitcom The Good Life, spoke about the important role RVS volunteers played hosting and cooking meals for social gatherings.

She said: “It’s giving comfort and time and fun, and just the fact you’re getting people together regularly through the year, it’s not just a special occasion, it’s actually many times a week that people are gathering together…and they are fed and become a unit, they make friends and they become a little community.”