The Prince of Wales and Duchess of Cornwall unveiled a plaque to mark the new name at a Newport hotel

The controversial move, in 2018, prompted complaints that there should have been a public consultation.

Quoted in a new book about the King, Lord Dafydd Elis-Thomas said the Prince assured him he knew nothing about the decision and was not consulted himself.

"He really wasn't happy at all," said the former Plaid Cymru leader.

In Charles the King and Wales, BBC Wales journalist Huw Thomas describes the bridge episode as the "most perilous moment" in Charles's time as Prince of Wales.

The same week as the ceremony to mark the renaming, in July 2018, Prince Charles toured Tretower Court and Castle, near Crickhowell, Powys, and met staff before having a conversation with Lord Elis-Thomas, whose report on the encounter is detailed in the book.

"It was during that visit that he turned to me and said: 'I want to tell you one thing, Lord Elis-Thomas. It's about this bridge.'

"'I just want to let you know that I knew nothing about it. That I was not consulted'.

"'Well, they were taking your name in vain, sir,' said I, or something like that. And we laughed. But he really wasn't happy at all."

There was also some criticism of the size of the new sign when it first appeared

Yet the Welsh secretary at the time, Alun Cairns, told the book's author he had tested the water on renaming the Second Severn Crossing in honour of the Prince of Wales in the summer of 2016, at the National Eisteddfod in Abergavenny, Monmouthshire.

Mr Cairns explained he had an informal conversation at the event with Grahame Davies, deputy private secretary to the Prince of Wales, who saw "merit in exploring this".

Prime Minister Theresa May and her transport secretary were "supportive", according to the Conservative MP, and, after three letters and two conversations with Carwyn Jones, the Labour Welsh first minister came onside.

The formalities then commenced, including asking for the Queen's permission, with her private secretary responding that Her Majesty was content with the idea.

The Second Severn Crossing opened in 1996

The book reports a UK government source saying Clarence House was "really up for" the Prince of Wales driving his Aston Martin sports car over the bridge to confirm the change of name, during early discussions.

Aston Martin had announced in 2016 it would open a new manufacturing centre in St Athan, Vale of Glamorgan.

The Prince marked the renaming, in July 2018, in a ceremony 25 miles away from the bridge, in a marquee at Newport's Celtic Manor Hotel.

According to the book, a senior UK civil servant, speaking on condition of anonymity, said the location made clear the Prince's feelings.

"He unveiled a new name for a bridge 25 miles from the bridge, and that says it all," the civil servant said.

Charles the King and Wales, by Huw Thomas, will be published by Parthian Books on 1 May.

Buckingham Palace was approached for comment.