The Queen has arrived at Buckingham Palace as she prepares to meet David Cameron.
He will formally tender his resignation, and will recommend that Theresa May - as leader of the largest party in the House of Commons - should be invited to form a government.
The Queen has tweeted to thank her fans for the "many messages of goodwill" she has received after her official 90th birthday celebrations.Read the full story ›
Buckingham Palace said the Queen had sent a personal message to President Obama, saying: "Prince Philip and I have been shocked by the events in Orlando.
"Our thoughts and prayers are with all those who have been affected."
Video report by ITV News' Royal Editor Tim Ewart:
Rain soaked revellers have celebrated the Queen's 90th birthday with a street party on the Mall for 10,000 well-wishers.
The Duke of Cambridge thanked the guests for not letting "a little rain spoil a good day out" and praised his "granny" as someone "we all look up to in good times and in challenging moments to set an example".
The Patron's Lunch was a fitting end to three days of events to mark the Queen's official 90th birthday.
Prince William has paid tribute to the Queen in an address to crowds at The Mall.
The Duke of Cambridge said the Royal Family has had plenty of reasons to celebrate since the Queen turned 90 in April, including her "strong health and relentless energy, her sharp wit and famous sense of humour".
He also paid tribute to the Queen's patronage of over 600 charities.
"We are here to celebrate the Queen as patron," he said. "Her commitment to the more than 600 charities to which she lends her name and support is unwavering. The Queen's pride in her role as your patron is absolute."
The Queen and the Duke of Edinburgh have greeted revellers lining The Mall.
They waved at well-wishers as they were driven down the route in an open-top motorcade.
Members of the Royal Family greeted revellers lining The Mall ahead of a parade to mark the Queen's 90th birthday.
The weather did not dampened people's spirits as the Duke and Duchess of Cambridge spoke with well wishers thronged behind barricades.
Prince Harry, the Earl and Countess of Wessex, and the Queen's grandson Peter Philips also shook hands with crowds standing in plastic ponchos along The Mall.
Revellers in plastic ponchos have descended on The Mall for the street party to celebrate the Queen's 90th birthday despite the wet weather.
Some 10,000 people will party on the famous London thoroughfare today.
There will be a carnival-style parade watched by the monarch, the Duke of Edinburgh, the Duke of Cambridge and Prince Harry.
Queen will be guest of honour at a huge street party on Sunday as three days of celebrations to mark her 90th come to a spectacular close.Read the full story ›