Michelle Obama has said she still feels "imposter syndrome", adding that "it never goes away", during a frank talk in London.

The former US First Lady was speaking at the Royal Festival Hall for a sold-out event that tens of thousands of people had tried to get tickets for.

She also recalled how the Queen had dismissed royal protocol as "rubbish" when the Obamas visited Windsor Castle.

She said she was panicking about how to act but the Queen said: "Just get in".

On Monday evening, Mrs Obama spoke to author Chimamanda Ngozi Adichie at the Southbank Centre to promote her new autobiography Becoming.

More than 40,000 people had reportedly tried to get tickets online when the event went on sale.

And last month, her memoir broke sales records to become the best-selling book released this year, just 15 days after being published.

Asked how she felt to be seen as a "symbol of hope", Mrs Obama told the live audience: "I still have a little imposter syndrome, it never goes away, that you're actually listening to me.

"It doesn't go away, that feeling that you shouldn't take me that seriously. What do I know? I share that with you because we all have doubts in our abilities, about our power and what that power is.

"If I'm giving people hope then that is a responsibility, so I have to make sure that I am accountable.

"We don't have any choice but to make sure we elders are giving our young people a reason to hope."

Imposter syndrome is a term used to describe feelings of insecurity or self-doubt, despite there being no evidence to support such a belief.

During her talk, Mrs Obama also recalled her and President Barack Obama's visit to meet the Queen.

"I had all this protocol buzzing in my head and I was like 'don't trip down the stairs and don't touch anybody, whatever you do'.

"And so the Queen says 'just get in, sit wherever' and she's telling you one thing and you're remembering protocol and she says 'Oh it's all rubbish, just get in'."

Earlier on Monday, Mrs Obama paid a return visit to Elizabeth Garrett Anderson secondary school in Islington, north London, which she visited in 2009, when her husband was US president.

Mr Obama served two terms in the White House. Mrs Obama is a lawyer and has led charity initiatives including Let Girls Learn, to promote education for girls around the world.

Her wisdom for young women?
At the Southbank Centre, Mrs Obama described her experience of black women being caricatured, saying: "The size of our hips, our style, our swag, it becomes co-opted but then we are demonised.

"My advice to young women in that you have to start by getting those demons out of your head. The questions I ask myself - 'am I good enough?' - that haunts us, because the messages that are sent from the time we are little is: maybe you are not, don't reach too high, don't talk too loud."

The former first lady said this was true for women and working-class people, and "profound for women of colour", as people in power tried to make them feel they did not "belong".

"Here is the secret," she added. "I have been at probably every powerful table that you can think of, I have worked at non-profits, I have been at foundations, I have worked in corporations, served on corporate boards, I have been at G-summits, I have sat in at the UN; they are not that smart."