Image caption The Queen's birthday is marked by the Trooping the Colour parade

The US State Department has been left red-cheeked after sending birthday greetings to the Queen a week early.

The diplomatic faux pas was admitted by spokesman PJ Crowley, who said: "Better a week early than a week late."

The message from US secretary of state Hillary Clinton on behalf of President Obama also honoured the "special relationship" between the US and UK.

Buckingham Palace said no offence had been taken, and that it had been a "genuine mistake".

In the premature message, Mrs Clinton said: "On this celebratory occasion, we pay tribute to the Queen's life and legacy and honour the special relationship between our two nations.

The statement noted how, "the young Princess Elizabeth helped rally her nation in the darkest days of that war, and she has remained a beacon of integrity and resolve ever since".

It said: "The United States has always been grateful for her friendship and example. Once again, I wish Queen Elizabeth II a very happy birthday and peace and prosperity for the people of the United Kingdom in the year to come."

The Queen's actual birthday is on 21 April - she turned 84 this year - but British monarchs celebrate a second "official" birthday on a Saturday in June decided by the government, this year on 12 June, culminating with the annual Trooping the Colour parade.