King Charles has thrown his support behind Australia in a landmark speech to the US Congress overnight.

Charles and Queen Camilla are in Washington for the second day of their four-day visit to mark the 250th anniversary of US independence.

'And we have agreed the most ambitious submarine programme in history – A.U.K.U.S. – in partnership with Australia, a country of which I am also immensely proud to serve as Sovereign,' the King told Congress.

'We do not embark on these remarkable endeavours together out of sentiment. We do so because they build greater shared resilience for the future, so making our citizens safer for generations to come.'

The King's remarks come just weeks after Donald Trump, who is understood to have been watching the speech from the White House, repeatedly blasted Australia for not joining the US-Israel led war in Iran.

Charles also underlined the importance of NATO in his speech, another military alliance Trump has repeatedly criticised, in a nod to the 25th anniversary of the 9/11 terrorist attack, when NATO's Article 5 was invoked for the first and only time.

'We answered the call together, as our people have done so for more than a century, shoulder to shoulder, through two World Wars, the Cold War, Afghanistan and moments that have defined our shared security,' Charles said.

'Today, Mr Speaker, that same, unyielding resolve is needed for the defence of Ukraine and her most courageous people, in order to secure a truly just and lasting peace.'

Charles also made a brief reference to Saturday's assassination attempt, when a shooter attempted to kill Trump and senior members of his administration at the White House Correspondents' Dinner, in a spirit of 'solidarity, sympathy and support', adding that 'such acts of violence will never succeed'.

He used his historic 20-minute speech to Congress to also reference the victims of sexual abuse, although he did not refer to the survivors of Jeffrey Epstein directly.

'In both of our countries, it is the very fact of our vibrant, diverse and free societies that gives us our collective strength, including to support victims of some of the ills that, so tragically, exist in both our societies today,' he said.

Concluding his speech, the King said that 'the story of the United Kingdom and the United States is, at its heart, a story of reconciliation, renewal and remarkable partnership'.

'From the bitter divisions of 250 years ago, we forged a friendship that has grown into one of the most consequential Alliances in human history,' he said.

'I pray with all my heart that our Alliance will continue to defend our shared values, with our partners in Europe and the Commonwealth, and across the world, and that we ignore the clarion calls to become ever more inward-looking.

'Mr Speaker, Mr Vice-President, distinguished ladies and gentlemen, America's words carry weight and meaning, as they have since Independence.

'The actions of this great Nation matter even more. President Lincoln understood this so well, with his reflection in the magisterial Gettysburg Address that the world may little note what we say, but will never forget what we do.

'And so, to the United States of America, on your 250th birthday, let our two countries rededicate ourselves to each other in the selfless service of our peoples and of all the peoples of the world.

'God bless the United States and the United Kingdom.'

Before the King's speech, Trump hosted the royals at the White House, where he made a series of gaffes, including telling Charles his late mother had a 'crush' on the monarch.

'I wonder what she's thinking right now,' he added.

Charles and Camilla will now head to New York for a day, where they are understood to be attending a tribute to the victims of 9/11, before returning to Washington for a formal goodbye with the President and First Lady.

They will then visit Virginia to attend an event in celebration of the 250th anniversary of US independence.