Thanks to a legacy from his mother, Harry will pocket the fortune as he celebrates his birthday on September 15.
And even though he will face a hefty tax bill, there will be more than enough left to keep the Prince happy, The Mirror reported.
Harry could have to fork out around 4 million pounds in duty, but tax experts say he could limit it to 3.6 million pounds if he gives cash to charities.
When William got his own 10 million pounds share of his mother's estate two years ago, 40 per cent went on inheritance tax.
"There is no way Harry would dodge the tax. There are few similarities between the average person and Harry and William but when it comes to tax, they also have to pay their way," a royal insider was quoted as saying by the daily.
"William got hit with a hefty bill when he turned 30 and so will Harry. His aides have to work out the most tax-efficient way for him to handle the money," the royal insider said.
Harry currently earns 38,847 pounds a year as an Army captain.
He was just 12 when Diana was killed in a Paris road crash in 1997.
She left 12,966,022 pounds which was reduced to 8,502,330 after death duties. But insiders believe the fund has now swelled to more than 20 million pounds thanks to clever investments by royal advisors.
The huge amount came from the settlement she got in her divorce from Prince Charles, shares, jewellery, cash, and personal items from her Kensington Palace home.
Diana shared her wealth equally between her two sons even though William will inherit his father's money-spinning Duchy of Cornwall when he becomes Prince of Wales.
Harry's huge windfall comes as Buckingham Palace accounts show the Queen will receive an inflation-busting five per cent rise in public funding to 37.9 million pounds next year due to record profits from the Crown Estate.
It has also emerged that the cost of doing up William and Kate's Kensington Palace home is costing taxpayers 4.5 million pounds.
Royal aides insist the monarchy still offers value for money at 56 pence per person a year.