The Princess of Wales has dug deep into the royal vaults to debut a $13 million tiara at a glittering state banquet at Windsor Castle.

Kate donned the rarely seen Queen Victoria’s Oriental Circlet Tiara for Wednesday night’s official dinner for the German President Frank-Walter Steinmeier and First Lady Elke Büdenbender on their three-day state visit to Britain.

She paired the astonishing piece of jewellery with a blue-sequinned Jenny Packham dress and earrings that once belonged to Queen Elizabeth II. The outfit was finished off withKate’s Royal Family Order and the Royal Victorian Order sash and star.

Jeweller Garrad says the Oriental Circlet Tiara was made for Queen Victoria in 1853 under the direction of her husband and consort, Prince Albert.

It was originally set with a collection of opals, one of Albert’s favourite gemstones, and 2600 diamonds. The opals were later replaced with rubies.

British media reports the tiara – which Victoria designated as an heirloom of the crown – is worth £6.5 million ($A13.1 million).

Its outing with Kate is the first time it has been seen publicly since 2006, when Queen Elizabeth wore it – the only time she did.

It is only the fifth tiara Kate has worn since joining the royal family in 2011, and her biggest yet. She wore the Cartier Halo tiara on her wedding day and has chosen the Queen Mary Lover’s Knot at least a dozen times since.

The princess has also worn the royal collection’s Lotus Flower tiara and Strathmore Rose tiara, although much less often.

Kensington Palace released a picture of Kate and husband Prince William ahead of the banquet in St George’s Hall.

There were more than 150 guests at Windsor Castle on Wednesday (British time) for the state banquet. They included a host of British and German celebrities and politicians, among them German supermodel Claudia Schiffer and her husband Sir Matthew Vaughn, Strictly Come Dancing judge Motsi Mabuse, who lives in Germany, former Everton midfielder Thomas Hitzlesperger, award-winning German composer Hans Zimmer and children’s book illustrator Axel Scheffler.

Kate in the Lovers Knot Tiara for a state banquet in 2022.

Earlier, Kate had joined husband Prince William to welcome Steinmeier and Büdenbender at Heathrow Airport. For that, the princess was clad in a designer coat described by media as “Prussian blue”.

She and William accompanied the German couple to Windsor Castle for their official welcome by the King and Queen.

It is the first state visit to Britain by a German leader in 27 years.

The King, meanwhile, used his speech at the dinner to say Britain and Germany are ready to “bolster Europe” against the threat of further Russian aggression and both nations “stand” with Ukraine.

He spoke as tensions between Europe and Russia appeared to intensity this week after President Vladimir Putin accused Kyiv’s European allies of sabotaging US-led efforts to end the war.

“With regard to shared defence, our German-British Bridging Battalion at Minden – which I visited during our state visit two years ago – is literally building bridges, a unique partnership at the heart of NATO,” the King said.

“More broadly, of course, the United Kingdom and Germany together stand with Ukraine and bolster Europe against the threat of further Russian aggression.”

Putin has rejected changes proposed by Ukraine and Europe to a draft peace plan as unacceptable, and he also said Moscow was “ready” for war with Europe.

The King told guests it was 36 years since the fall of the Berlin Wall – “a momentous event in human history”.

“The tumultuous period of political, social and technological change that happened after that has tested our values. Many have found that unsettling and even frightening. Fear can lead to anger and resentmen,” he said.

“But the United Kingdom and Germany are united in a continued belief in democracy, freedom and the rule of law.”

Elsewhere, Kate will be out again on Friday as she hosts her fifth annual “Together at Christmas” Christmas carol at Westminster Abbey.