The union flag flying above Buckingham Palace, the Queen's official London residence, was lowered to half-mast as a mark of respect.
A Buckingham Palace spokesman said: "It's in line with Government policy advised by the DCMS. Union flags (at Government departments) at half-mast on days of significant deaths."
Other royal residences flying the union flag like Clarence House, the Princes of Wales' London home, also lowered their banners.
The death of Baroness Thatcher dominates all of the UK newspaper front pages today.
Baroness Thatcher, who was Britain's prime minister for over a decade, has died today aged 87 after suffering a stroke, her family said.