Prince William has turned 25 and reached the age at which he is entitled to part of the inheritance left to him by his mother Princess Diana.
It may be his birthday but the future King still has to work
The prince can now access the interest and other investment income which is generated on his £6.5m share, estimated to be £250,000 to £300,000 a year.
But sources have said the future King will not, for now, touch the money.
The milestone birthday also grants Prince William the right to marry without consent from the Queen.
When Princess Diana died a decade ago, she left an estate of £21m.
More than £8m was paid in inheritance tax, leaving £12,966,022, which was split equally between William and younger brother Prince Harry.
Each £6.5m share is estimated to have grown to around £9m after investment.
Birthday on duty
The two princes each have to wait until they reach 30 before they can ask for the entire sum due to them.
Prince William already earns around £22,000 as a Cornet in the Blues and Royals and receives money from the Prince of Wales to pay for his accommodation, staff and polo playing.
He is also believed to have access to funds left in trust for him by his great grandmother the Queen Mother.
But rather than spending his cash, the Prince William will spend his birthday training with the Army at Lulworth in Dorset.
Although he is currently single - he recently had a high-profile split from Kate Middleton, who he met at university - the Prince is now entitled to wed without asking his grandmother's permission.
Under the Royal Marriages Act (1772), a member of the Royal Family may, at the age of 25, signify to the Privy Council his or her intention to marry without the consent of the Sovereign.
The marriage can then lawfully take place unless both Houses of Parliament disapprove.