Is this Kate Middleton's most revealing dress ever?

She has drawn criticism for playing it safe in the fashion stakes, but the Duchess of Cambridge proved she is not afraid of pushing the boundaries as she attended a glittering state banquet held in honour of the King and Queen of Spain.

Kate, 35, looked sensational in a dusty pink custom-made Marchesa gown as she joined Prince William, the Queen, the Duke of Edinburgh and other senior royals to welcome King Felipe VI and his wife Queen Letizia to Buckingham Palace, the Daily Mail reports.

The breathtaking floor-length dress featured a plunging V-neck in scalloped lace, which was a surprisingly revealing choice for the Duchess and a departure from the conservative cuts she usually favours.

Kate drew further attention to her decolletage with an eye-catching diamond and ruby drop necklace, the ruby boucheron bandeau, which once belonged to the Queen Mother. The jewels fell just inches above the lowest point of the V-neck.

The garment's sheer bell sleeves added to its drama, which was offset by its soft colour and delicate lace fabric.

In keeping with royal tradition, Kate donned a tiara for the state dinner. The diamond and pearl Cambridge Lover's Knot tiara, which she wore to her first state banquet last year, was a piece beloved by William's late mother, Diana, Princess of Wales.

Kate finished the look with a favourite pair of diamond and pearl drop earrings.

The gown is markedly more daring than the dresses Kate chose to wear to her last two evening events, which both took place in March this year.

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For an event at the Ambassador's residence, she dazzled in an ice-blue Jenny Packham dress. While it caught the light with the beaded detailing, the sheer overlay meant very little skin was on show.

Later in the month, Kate stepped out for a fundraising event at the National Portrait Gallery in London in a glamorous forest green floor length gown by Temperley London, one of her favourite designers.

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However while once again it was an elegant choice for the Duchess, the long sleeves and high neck meant it was anything but risque.

It might be that Kate wanted to step up the style stakes tonight as she was coming face to face with Queen Letizia, whom she is regularly pitted against on best-dressed lists.

Never afraid of making a statement, Letizia, 44, opted for an off-the-shoulder red gown, which she paired with a glittering tiara that once belonged to her mother-in-law, Queen Sofia.

Meanwhile, the Queen donned a white gown and a Brazilian Aquamarine Tiara, which is a favourite of the Queen's dresser, Angela Kelly, who describes it as "majestic in appearance but cool and calming".

As for Camilla, the stand-out part of her outfit was the glittering Greville tiara, on loan from the Queen who was left the tiara by her late mother.

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Guests at the Spanish state banquet dined on medallion of Scottish beef and truffles in a Madeira sauce as they gathered in the opulently decorated Buckingham Palace ballroom as the Countess of Wessex's String Orchestra provided the musical entertainment - including curious tunes such as the theme from the Bond film Skyfall and Coldplay's Viva La Vida.

While it's understood to be Prince Philip's last-ever state banquet before he retires this Autumn, it was Prince Harry's first as he moves into a more full-time royal position. The Duke of Cambridge attended his first Palace banquet last year, while Prince Charles was 20 at his first state banquet in the London residence.

The white tie Diplomatic Reception is the main social event of the year in London for the diplomatic community and reflects the Queen's importance in terms of the UK's international relations.

The Queen's diamond and ruby necklace worn by Kate

The opulent collar Kate wore tonight, the Ruby and Diamond Floral Bandeau, was a wedding gift from the Queen's parents.

Bought by Margaret Greville, a close friend of her mother's, from the jeweller Boucheron in 1907, it was bequeathed to the Queen Mother, along with several other jewels, on Greville's death in 1942.

The intricate piece is set in silver and gold, in an unusual V-shape and features several sizeable rubies.

The Queen, who has remarkably slim shoulders, tends to shorten her necklaces to make them sit higher on her neck, and did so here by removing two small floral pieces from the sides.

She wore it often in her youth, most memorably at the Royal Variety Performance in 1962, paired with her favourite Girls of Great Britain and Ireland tiara.

This article was first published on Daily Mail and is republished here with permission.