Reform UK's Zia Yusuf has vowed to ban New York's Left-wing mayor from Britain after he urged King Charles to return the Koh-i-Noor diamond to India.
The famous 105-carat diamond - one of the largest cut diamonds in the world - is part of the Crown Jewels held at the Tower of London.
But its ownership has been disputed by India, who claim that it was stolen during British rule.
Ahead of meeting Charles during his state visit to the US this week, Zohran Mamdani - who was elected as New York mayor last year - vowed to raise the issue.
'If I were to speak to the King separately from [the purpose of the event], I would probably encourage him to return the Koh-i-Noor diamond,' he said.
The socialist mayor, known for his anti-colonialist views, was subsequently pictured sharing a laugh with Charles when they met in New York on Wednesday.
But Mr Yusuf, who would be in charge of the Home Office if Reform won power, lashed out at Mr Mamdani's remarks and accused him of 'insulting' the King.
'Mamdani may be looting New Yorkers, but he will have no joy in Britain,' he posted on X. 'Unless he retracts this insult to our King, as Home Secretary I would ban him from entering Britain.'
Zohran Mamdani, the socialist mayor of New York, was pictured sharing a laugh with King Charles when they met on Wednesday
The famous 105-carat Koh-i-Noor diamond was set in the Crown of Queen Elizabeth, the Queen Mother, in 1937
Reform UK's Zia Yusuf, who would be in charge of the Home Office if Reform won power, lashed out at Mr Mamdani's remarks and accused him of 'insulting' the King
Mr Yusuf added: 'This beautiful diamond is currently on display in the Tower of London. That is where it will stay.'
However, the Reform politician was questioned over his party's commitment to 'free speech' in the wake of his social media outburst.
Shockat Adam, the independent MP for Leicester South, replied: 'No, Mohamed Yusuf “loot” is what the empire did in India, which is why the word originated.
'And what's with this banning? I thought Reform were champions of free speech?'
Mr Mamdani met with King Charles and Queen Camilla during their visit to the 9/11 memorial in New York.
The King and Queen laid a bouquet of white roses at the edge of the memorial's south pool during a moment of reflection.
At the age of 34, Mr Mamdani is the youngest mayor of New York in more than a century and also the first Muslim to hold the role.
He centred his election campaign on bringing down costs for New Yorkers through rent freezes, the creation of city-owned grocery stores, the introduction of free buses, and universal free childcare.
Mr Mamdani also vowed to 'tax the rich' with levy increases for businesses and the wealthiest 1 per cent of the city's population.
The Koh-I-Noor diamond is thought to originate from the Golconda mines in central southern India.
It has had many previous owners, including Mughal Emperors, Shahs of Iran, Emirs of Afghanistan, and Sikh Maharajahs.
The East India Company took the jewel from deposed 10-year-old Maharajah Duleep Singh in 1849, as a condition of the Treaty of Lahore.
This marked the end of the Anglo-Sikh Wars in the Punjab, in present day northern India and eastern Pakistan. The Treaty specified that the jewel be surrendered to Queen Victoria.
The Koh-i-Noor diamond was once much larger but was re-cut in 1852 to improve its brilliance and conform to contemporary European tastes. It was set in the Crown of Queen Elizabeth, the Queen Mother, in 1937.