The Duke of Sussex has said he is "taking a leap of faith" in stepping back from being a senior royal, adding: "There really was no other option."
Speaking at an event on Sunday evening, Prince Harry said he and Meghan had hoped to continue serving the Queen, but without public funding.
"Unfortunately, that wasn't possible," he said.
It was his first speech since the couple said they wanted to stand down from being full-time working royals.
The prince said he had found "the love and happiness that I had hoped for all my life" with Meghan, but he wanted to make it clear they were "not walking away".
"The UK is my home and a place that I love, that will never change," he said.
Prince Harry said it was a sign of the pressures he was feeling that he would "step my family back from all I have ever known" in search of "a more peaceful life".
Earlier this month, Prince Harry and Meghan said they intended "to step back as 'senior' members of the Royal Family and work to become financially independent".
On Saturday, Buckingham Palace announced that from the spring they will stop using their HRH titles and withdraw from royal duties, including official military appointments.
And on Monday Prince Harry was pictured at the UK-Africa Investment Summit in London, where he held a number of private meetings, including with Prime Minister Boris Johnson.
The Duke and Duchess of Cambridge hosted an evening reception at Buckingham Palace for heads of government, ministers, business leaders and members of NGOs attending the summit.
It was the first time the duke and duchess had hosted a reception for world leaders on behalf of the Queen.
Prince Harry did not attend, with BBC royal correspondent, Nicholas Witchell, saying he is believed to have left on a flight for Canada from Heathrow airport.
Prince William and Catherine were joined at the reception by senior royals including the Princess Royal and the Earl and Countess of Wessex.
Beginning his speech at a fund-raising reception in central London for Sentebale, the charity he co-founded which helps children living with HIV in southern Africa, he said: "I can only imagine what you may have heard and perhaps read over the past few weeks.
"So I want you to hear the truth from me as much as I can share, not as a prince or a duke but as Harry."
'A powerful force'
During his address, the prince said he would always have "the utmost respect for my grandmother, my commander in chief".
"Our hope was to continue serving the Queen, the Commonwealth, and my military associations, but without public funding. Unfortunately, that wasn't possible," he said.
"I've accepted this, knowing that it doesn't change who I am or how committed I am."
Prince Harry and Meghan have both spoken about the difficulties of royal life and media attention, with the duke saying he feared his wife would fall victim to "the same powerful forces" that led to his mother's death.
He told the audience at the reception for Sentebale, which he founded to continue Princess Diana's legacy in supporting those with HIV and Aids, that he felt they took him "under your wing" after she died.
"You've looked out for me for so long, but the media is a powerful force, and my hope is one day our collective support for each other can be more powerful because this is so much bigger than just us," he said.
Sentebale held an event on Sunday 19th January 2020, hosted by The Caring Foundation, to raise funds for Sentebale’s vital work supporting young people affected by HIV in southern Africa. pic.twitter.com/bxL2RqhiNS— @Sentebale (@Sentebale) January 19, 2020
As part of a deal finalised on Saturday between the Queen, senior royals, and the couple, Harry and Meghan agreed they will no longer formally represent the monarch.
However, the statement by Buckingham Palace said they would continue to maintain their private patronages and associations.
Prince Harry said in his speech that he and Meghan "will continue to lead a life of service".
"I will continue to be the same man who holds his country dear and dedicates his life to supporting the causes, charities and military communities that are so important to me," he said.
Johnny Hornby, chairman of Sentebale, said the new arrangements would not affect the prince's work for the charity. "We don't need - from Sentebale's perspective - his title, we just need his time and his passion," he told the BBC.
There are two big messages in this speech. The first is to deal with the "Meghan myth" - the idea that the Duchess of Sussex is at the root of the couple's desire to lead a different life.
Harry speaks of "many months" of discussions over how to deal with the challenges of "many years"; he's making it clear that he was unhappy with his role long before Meghan entered his life
And he talks about the decision that "I" made, a decision "I" did not make lightly. He stresses that this was his call, though it was clearly one that they came to together.
The second message is that he wanted to continue in some sort of a royal role; "unfortunately," he says "that wasn't possible."
Both sides - the Sussexes and the Palace - thought at the beginning of negotiations that such a half-in, half-out role might be possible. But the tension between a royal life and an independent life was too great; the contradictions and possible conflicts of interest were too many.
Harry may or may not believe that to be true. But he wants to let people know that his desire, at least, was to continue to serve.
Former Liberal Democrat MP Norman Baker, who is the author of a critical book about the Royal Family, said the public could end up paying for part of the Prince of Wales' ongoing financial support for his son.
Mr Baker told BBC Radio 4's Today programme that the Queen already offset support for family members against the tax bill for the Duchy of Lancaster, the sovereign's estate.
Much of Prince Harry's funding comes from his father's estate, the Duchy of Cornwall.
Mr Baker called for Prince Charles to say how he will support Harry and to publicly guarantee there would be no loss to the taxpayer through a reduction in his tax liability.
The former MP also called for the Commons public accounts committee to investigate royal finances.
Journalist and royal author Robert Hardman said the agreement with the Queen meant the duke and duchess's Sussex Royal brand, which they applied to trademark last year, is not "sustainable".
"The whole thrust of what has been agreed with the Queen is they won't be trading on their royal credentials," he said.
In Prince Harry's speech, posted on the couple's Instagram account, he said that when he and Meghan were married "we were excited, we were hopeful, and we were here to serve".
"For those reasons, it brings me great sadness that it has come to this.
"The decision that I have made for my wife and I to step back is not one I made lightly," he said.
"It was so many months of talks after so many years of challenges and I know I haven't always got it right, but as far as this goes there really was no other option."
The couple said they plan to divide their time between the UK and Canada, after they spent six weeks on Vancouver Island with their son Archie over Christmas.
The prince told attendees it was a "privilege... to feel your excitement for our son Archie, who saw snow for the first time the other day and thought it was bloody brilliant!"
The duchess is currently staying on Canada's west coast with her son, after briefly returning to the UK earlier this month.
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