The strange case of Lady Thatcher and Her Majesty's coat of arms

While in power, critics often accused Margaret Thatcher of behaving as imperiously as any monarch.

Now, in a move which may cause mild irritation at Buckingham Palace, the former prime minister has gone one step further by ditching her own coat of arms - and adopting the Royal Arms as her official letterhead.

In recent weeks letters have been sent from her private office bearing the design which, in its fullest version, is for use by the Sovereign alone.

The coat of arms contains a central shield bearing the arms of England, Scotland and Ireland, surrounded by the garter, supported by a lion and unicorn. The motto Dieu et mon Droit is below.

Though the Queen's personal version is fuller, there are understandable grounds for confusion as the shield, mottoes, lion, unicorn and garter are common to both.

The form being used by Baroness Thatcher is that used by formal state bodies; secretaries and departments of state, and the Houses of Parliament, for example.

And, but for the substitution of the words Margaret Thatcher, it is also identical to that of the office of 10 Downing Street - perhaps a sign that even after more than six years the baroness still finds it hard to adjust to non-prime ministerial life.

The normal House of Lords logo used by peers places the Arms inside an ellipse, together with the words "House of Lords", making clear the state body to which the use of the Arms relates.

Many peers simply have their names printed beneath the logo on Lords paper rather than have the whole letterhead printed for them.

The heraldry expert Thomas Woodcock, Somerset Herald at the College of Arms, dismissed a suggestion that as a Lady Companion of the Order of the Garter she could use the Royal Arms unadorned. "She would use her own arms as a Companion of the Order of the Garter and not the Arms of the Sovereign," he said.

Of the head on her notepaper, he said: "Well, it's certainly the Royal Arms. All I can say is, I find it very odd. I can't think of any reason why they should be there."

Meanwhile, the ceremonial figure of Black Rod, asked if Lady Thatcher's use of the Royal Arms broke the rules, said: "Well, I'm ... not making any comment on that at all."

The letterheads of other former prime ministers do not allude to their former status: Sir Edward Heath uses a simple House of Commons portcullis and a plain typeface, while Lord Callaghan simply types his name beside the House of Lords logo.

But Lady Thatcher's own rather quirky design, an unusual mixture of binoculars, weighing scales, and keys, seems to have been quietly dropped for grander things.