The Duke and Duchess of Cambridge have been criticised for failing to “make sure they are beyond reproach” after they were pictured breaching the Rule of Six with other royals.

William and Kate and their family were photographed together with the Earl and Countess of Wessex and their children during a night-time woodland walk on the Queen’s Sandringham estate in Norfolk.

Norfolk is in Tier 2 and, under restrictions, up to six people can meet outside – known as the Rule of Six.

The two families – nine in total – were pictured together on Sunday as they were given a preview of Luminate – an event open to the public where visitors follow a light trail through the Country Park on the estate.

A source suggested the two families meeting up was inadvertent, due to the presence of young children and bottlenecks on the walk.

Graham Smith, from the organisation Republic – which campaigns for an elected head of state, said: “I think the royals should not be relying on the inability of the authorities to enforce the law, they have a responsibility to show a higher standard and to make sure they are beyond reproach.”

He added: “When we see Sky staff rightly being sanctioned for breaching the rules, people who are in the public eye and have to set an example, then there ought to be some way of doing the same with William and Kate.”

In one image published in the Daily Mail, William leads the group with his youngest son, Prince Louis, on his shoulders. Prince George and Princess Charlotte are close by, with Kate behind them and Sophie and her son, Viscount Severn, alongside.

Lady Louise follows behind her mother, the countess, with the earl at the back, making nine people in total.

The event is billed on the estate’s website as a “spectacular, illuminated trail, full of wonder and intrigue, to delight and enthral your senses”.

The Cambridges and the Wessexes arrived separately, but were seen “mingling” together several times, according to a member of the public who took photographs on their smartphone.

The Earl and Countess of Wessex live in Bagshot Park in Surrey, which was put into Tier 4 on Sunday, with residents of this tier asked not to leave home unless they have a reasonable excuse.

But it is understood the couple and their family had left their home when it was still in Tier 2.

William and Kate live close to Sandringham in their country home, Anmer Hall, and it is thought they moved to the residence from their London home at Kensington Palace after the school holidays began.

A source at Sandringham said: “The two families were given separate consecutive slots to visit the trail just before it opened to the general public. They arrived and departed in their own family groups.

“As anyone with young children will know, there were moments on the 90-minute walk where it was difficult to keep the two family groups apart, particularly at bottlenecks on the trail.”