Boris Johnson made an apparent dig at Nicola Sturgeon after she seemed to suggest Prince William and Kate Middleton had broken Covid rules with a train tour to Scotland.

The Duke and Duchess of Cambridge were in Cardiff on Tuesday for the final leg of a three-day trip to meet NHS staff, voluntary workers and others who have been helping combat coronavirus across the UK.

But under current Scottish regulations, it is illegal to travel between England and Scotland without a reasonable excuse and Sturgeon questioned on Monday whether the tour was necessary.

“The Scottish government was advised about the intention to visit, and we made sure that the royal household were aware, as you would expect, of the restrictions in place in Scotland so that could inform both the decision and the planning of the visit,” the first minister had said.

After initially saying the Cambridges’ tour was a “matter for the palace”, No.10 issued a new statement saying Johnson welcomed the “morale boost” it would provide.

A No.10 spokesperson said: “The PM is delighted to see the warm reception the Duke and Duchess of Cambridge have received on their hugely valuable train tour of England, Scotland and Wales.

“The tour will be a welcome morale boost to frontline workers who have done so much during the pandemic. 

It is understood that the Cambridges believed their trip to Edinburgh complied with the Covid regulations because their journey was “work-related” rather than a social trip.

The Welsh government had also raised its own concerns about the royal trip and Covid rules.

Welsh health minister Vaughan Gething told the BBC: “I’d rather that no one was having unnecessary visits and people always have divisive views about the monarchy, but their visit isn’t an excuse for people to say that they are confused about what they are being asked to do.”

The royal train tour has seen the couple visit a FareShare community food programme in Manchester that has been supported by footballer Marcus Rashford.

In a meeting with students in Cardiff, William admitted that his own family’s plans for Christmas were still up in the air.

“It is so difficult. We are still trying to make plans. It’s difficult to know what to do for the best.”

The Queen has decided to stay in Windsor for the festive break, the first time in decades she has not been at Sandringham.