The 74-year-old actress - who starred as the British monarch in 2006's 'The Queen' - has admitted she completely forgot to follow royal protocol when she was invited to have tea with the Queen.
She recalled: "She invited me for tea. I thought it was going to be in a room with 200 other people, which it often is. I've met her once before and it was in a room with 200 other people.
"So I said, 'Oh well I can manage that.' So it was at the horsey place and the message came to me that the Queen would like to invite you to tea, she knows you're here.
"So I walk in, and there's like eight people sitting around a table. Prince Philip, the Queen, a Sheik of somewhere or other and a couple of horsey people. I know absolutely nothing about horses, at all, and the Queen knows everything about horses."
Mirren explained that she was "desperately trying to make polite conversation and it's just coming out like gobbledygook".
And things quickly began to spiral out of control for the Oscar-winning actress.
Speaking at the 'Helen Mirren and Hawk Koch Discuss The Magic of Movies' event, Mirren continued: "Then I got a cup of tea and the Queen, in particular, was having a very intense conversation about what's in the sandwiches with Prince Philip, very important. 'What is this? What's in here?'
"The milk is on the other side of Prince Philip and I want some milk in my tea, but my brain goes completely dead and I cannot remember how to address Prince Philip.
"I mean, is it sir? Is it your majesty? Is it your highness? Is it rude to ask him to pass the milk? Or should I just ask for a lackey? I finished up not having any milk. I just couldn't sort it out."
Mirren still relished the experience of meeting the royals, despite the awkwardness.
She said, according to People: "It's a lesson in embarrassment, but they were lovely, they were utterly gracious."