AceShowbiz - Emma Thompson was left red-faced when she was presented with a damehood by Prince William last year after forgetting to curtsy for the British royal.

The "Love Actually" star was included in Queen Elizabeth II's 2018 Birthday Honours list, and she was invited to Buckingham Palace to receive the medal from the monarch's grandson.

Thompson has known the Prince since he was a child, and her familiarity with the now-37-year-old caused her to forget all about the etiquette rules the honourees had been given moments before the investiture ceremony.

"The etiquette is extremely tough... There's a rehearsal...You have to walk and then you have to turn, and then you have to curtsy, which of course, I forgot...!" she cringed on U.S. morning show "Live with Kelly and Ryan".

"It just didn't occur to me. I completely forgot immediately."

To make matters worse, the Prince had to politely decline Thompson's request to give him a friendly kiss as they greeted one another: "So instead, I rushed at him and said, 'Hello darling, how are you? Can I kiss you...?'."

Prince William had to explain such behaviour was not suitable for the occasion to avoid others from wanting to get too close to him, too.

"He said, 'No, don't do that, otherwise everyone will want one!'," Thompson recalled. "So I said, 'No, you're right, absolutely,' so I restrained myself, which, as you know, is not like me!".

The actress then remembered how she was supposed to have greeted William, so she tried to make amends for her blunder as she left.

"Then I went backwards and sort of did a terrible curtsy and then went off!" she shared.

It wasn't the only awkward moment of Thompson's presentation - she previously revealed Prince William had accidentally pinned her nipple - twice - as he affixed two badges to her lapel.