DUP Party Leader, Arlene Foster takes to the stage for her speech in the party's annual conference at the La Mon hotel in Belfast. Michael Cooper/PA Wire

Arlene Foster appeared to be in such a rush to send her congratulations to Prince Harry on his engagement to actress Meghan Markle that she got the wrong brother.

The DUP leader tweeted congratulations to Prince William instead.

While Prince William is no doubt delighted for his brother he is married to Kate with two children and another one on the way.

The tweet was quickly deleted and a replaced with congratulations to Prince Harry.

Congratulations to HRH Prince Harry and Megan Markle on their engagement. Wonderful news this morning! pic.twitter.com/vwYP6GNNN6 — Arlene Foster (@DUPleader) November 27, 2017

Several hours later Arlene Foster was back on Twitter to explain the mistake.

Apologies to TRHs Princes William & Harry for tweet error on my account earlier. I stopped tweeting personally a long time ago. Genuine typo by a member of staff. Guilty of tweeting too fast. There goes any chance of an invite!! :-) AF — Arlene Foster (@DUPleader) November 27, 2017

After the engagement was announced Harry's brother William and sister-in-law Catherine said: "We are very excited for Harry and Meghan. It has been wonderful getting to know Meghan and to see how happy she and Harry are together."

Prince Harry and his bride-to-be showed off the engagement ring when they posed for their first photographs since announcing their impending nuptials.

Prince Harry and Meghan Markle in the Sunken Garden at Kensington Palace, London, after the announcement of their engagement. Picture by Dominic Lipinski, PA Wire

The newly engaged couple

The Duke and Duchess of Cambridge appeared together the same day at St James's Palace in 2010, with Kate wearing the sapphire and diamond engagement ring that belonged to William's mother, Diana, Princess of Wales.

A date will have already been fixed. Royal aides are said to have synced royal diaries a while ago to find a suitable day and make sure the Queen, the Prince of Wales and the Duke and Duchess of Cambridge and other royals could all attend.

Royal weddings usually take place only a few months after an engagement announcement.

March is the most likely month, leaving enough time to ensure pregnant Kate is not about to give birth to her third child.

The venue will also have been decided and the Archbishop of Canterbury is expected to perform the ceremony.

St George's Chapel at Windsor Castle is a good bet. The 15th century church is grand, but will allow for a smaller scale royal wedding compared with Westminster Abbey and St Paul's Cathedral.

The Metropolitan Police will start planning immediately for the high-profile event. The current threat level for international terrorism in the UK is severe.

Extra officers will be drafted in to patrol the streets on the day amid tight security surrounding the chosen venue.

Ms Markle will also have been assigned a royal bodyguard from the Metropolitan Police's Royalty and Specialist Protection Command.

Bridal-wear designers around the world will be hoping to get the call for this royal wedding.

Ms Markle will no doubt already have an idea of what she wants.

The actress appeared in an Anne Barge ballgown wedding dress for her role as Rachel Zane in Suits, and her own first wedding to TV producer Trevor Engleson saw her wear a sleek, strapless, floor-length white dress with diamante detail.

Her stylist and close friend Jessica Mulroney runs the bridal store Kleinfeld Hudson's Bay in Toronto, so it is likely Ms Mulroney will be Ms Markle's first port of call for help picking out dress amid great secrecy.

Harry and his fiancee will have already drawn up a guest list, but there will also be a number of dignitaries they have to invite.

The prince's grandmother, the Queen will, however, be on hand to help.

William revealed after his 2011 wedding: "I went to (the Queen) and said 'Listen, I've got this list, not one person I know. What do I do?'

"And she went 'Get rid of it. Start from your friends and then we'll add those we need to in due course. It's your day."'