The King's arrival in Douglas on Tuesday drew thousands of spectators - but it was the royal car carrying His Majesty that left many asking the same question: where was the number plate?

Unlike every other vehicle on Manx roads, the King's official State car is exempt from displaying registration plates when carrying the Sovereign on official duties.

The vehicle, which is understood to have been brought to the Isle of Man aboard a Steam Packet sailing ahead of the royal visit, was the Bentley State Limousine – one of only two built by Bentley to mark Queen Elizabeth II's Golden Jubilee in 2002.

The armoured limousines remain the monarch's principal ceremonial vehicles and are now used by King Charles for major public engagements.

Finished in the distinctive royal claret and black livery, the Bentley is instantly recognisable for another reason – it has no front or rear registration plates.

The exemption is a long-standing constitutional convention. Because the Sovereign is the legal embodiment of the Crown, the official State cars used personally by the monarch are not required to display registration plates, making them unique on UK and Isle of Man roads.

The convention applies only to the King's official State vehicles, not to other members of the Royal Family or to the King's private cars, which are registered in the normal way.

The Bentley itself is far from an ordinary luxury saloon.

It features armour plating, blast-resistant construction, sealed windows and reinforced tyres, while its enlarged rear doors and raised roofline are designed to allow the monarch to enter and leave the vehicle with ease while also ensuring he can be clearly seen by crowds.

The distinctive limousine formed part of the King's motorcade as he travelled between engagements during his first official visit to the island as Lord of Mann.

His itinerary included addressing a special sitting of Tynwald, greeting well-wishers outside the Legislative Buildings, attending a reception at Government House, riding aboard the Douglas Bay Horse Tramway and visiting Heroes on the Water at Port Soderick before departing the island later that afternoon.