A lunch fit for a king? That's half an avocado for Charles... and he definitely doesn't smash it like a millennial!

King Charles has started to eat lunch for the first time – and The Mail on Sunday can reveal his meal of choice is the millennial's favourite, the avocado.

The 75-year-old monarch has spent a lifetime skipping lunch, with a source saying he regarded it as a 'luxury' and that there was not enough time in his busy schedule to allow such an indulgence.

Now, on the orders of his wife, aides and doctors, he has grudgingly started eating in the middle of the day to keep his strength up.

But the King, who is recovering from cancer, insists on eating only the healthiest – and trendiest – snack there is. A source said: 'With some reluctance, he now has something to eat at lunchtime – a snack, really. He now eats half an avocado to sustain him through the day. It's important, particularly if you have got an illness.'

Charles is a healthy eater, insisting on organic produce. Eggs are a particular favourite. He told the BBC three years ago how he abstains from meat and fish on two days of the week, and that he also avoids dairy on one of those days.

Avocados are a so-called superfood, known for being dense in calories and a healthy way to incorporate energy-fuelling 'good fat' into a diet. They are now virtually part of the nation's culture wars after becoming hugely popular with that much-criticised generational cohort, the millennials – born between 1981 and 1996.

Nigella Lawson and London restaurant The River Cafe are both widely blamed for the omnipresent snack of choice, avocado on toast. But the mastermind behind the phenomenon seems instead to have been Australian restaurateur, Bill Granger. He served his first plate of smashed avocado on toast three decades ago in a cafe in Sydney – little knowing he was sparking a worldwide revolution.

A love of avocado on toast has somehow become synonymous with millennials, and, by extension, the workshy. There was even an improbable controversy about how millennials would be much better placed to buy the homes they felt they were being priced out of by older 'boomers' if only they would rein in their prodigious outlays on the snack at hipster cafes.

But Charles' addition of them into his diet includes none of the carbohydrates that come with toast. He prefers them on their own and they are eaten only for utilitarian purposes.

Last week, the King's food-critic stepson Tom Parker Bowles gave an insight into Charles' economical approach to meals. 'Everything is recycled,' he said. 'If anything is left over, that will be made into something else or appear the next day. Nothing's thrown out.' His latest book, Cooking And The Crown, details the history of food in the royal family. He revealed that in colder months, his mother Queen Camilla has porridge every day and keeps her lunches light – chicken broth or smoked salmon – while Charles eats dried fruit and honey for breakfast and has preferred afternoon tea to lunch.

Mr Parker Bowles said: 'Tea has always been a spectacle – and an important and substantial meal in itself – for the royals. Everyone congregates for tea.

'And it's not just cakes and biscuits and crumpets and sandwiches. You might even get poached eggs. If you are staying in Scotland, it's quite dangerous, because you have a cooked breakfast, you have lunch, you have tea and you have dinner.'

'He eats them only for utilitarian purposes'