A huge chunk - around £150million - will be needed to deploy thousands of police officers and protection squads, insiders reveal.
The rest goes on staging the ceremony and three days of celebrations. The eye-watering cost to taxpayers is despite the King's wishes for a slimmed-down, cost-effective monarchy.
It cost Britain £1.57million to stage the Queen's Coronation in 1953 around £47million in today's money.
But ever-increasing security threats and a series of planned domestic protests have sent costs for Saturday's event - codenamed Operation Golden Orb - soaring way above earlier estimates of £100million.
Home Office sources revealed last night that a special air traffic control schedule has been drawn up for dignitaries flying in.
Security will be tightened at landing spots, which include RAF Northolt in Middlesex and RAF Biggleswade, Beds, from Thursday.
It comes amid fears that climate change protesters could target the private jet arrivals.
A major operation to stop activists sabotaging Coronation Day itself will see huge numbers of armed officers on the streets, snipers on rooftops, and police drones monitoring crowds for suspicious activity.
Groups like Just Stop Oil and Republic could team up to cause disruption.
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