President Barack Obama said the relationship between the US and Britain is "the strongest that it has ever been" as he welcomed David Cameron to Washington in a lavish ceremony at the White House.

On a sun-drenched South Lawn, packed with 7,000 guests - including children from both nations - the Prime Minister basked in the warmth of the friendship offered by the US president.

Mr Cameron returned it in kind, hailing examples of Anglo-American co-operation in war, diplomacy, science and business and describing Britain and the US together as "the united states of liberty and enterprise".

The extravagant ceremony, complete with 19-gun salute, a review of troops and marching fifers in colonial-era uniforms, was a far cry from the more low-key welcome offered to Mr Cameron's predecessor Gordon Brown shortly after Mr Obama's arrival at the White House, when the new president was keen to signal that his interests were directed towards Asia.

Now seeking re-election, with several years' experience of military co-operation with the UK in Afghanistan and Libya, Mr Obama was emphatic about the enduring value of what he terms the transatlantic "essential relationship".

"Our world has been transformed over and over and it will be again," said the president. "Yet through the grand sweep of history, through all its twists and turns, there is one constant - the rock solid alliance between the United States and the United Kingdom.

"The reason is simple - we stand together, we work together, we bleed together and we build together, in good times and in bad, because when we do, our nations are more secure, our people are more prosperous, our world is a safer and better and more just place." He added: "Our alliance is essential, it is indispensable to the prosperity and security that we seek not only for our own citizens but for people around the world."

He won laughter from the Prime Minister and the crowd by slipping into British English to tell Mr Cameron: "We are chuffed to bits that you are here, I'm looking forward to a great natter, I'm confident that we are going to keep the relationship between our two great nations absolutely top notch."

Mr Cameron responded that the relationship between the US and UK had produced "the most powerful partnership for progress that the world has ever seen". Britons and Americans were "kindred spirits" with a belief in creativity, innovation and risk-taking, who were ready to back their words with actions, he added.

"When the chips are down, Britain and America know we can always count on each other, because we are allies, not just prepared to say the right thing, but to do the right thing and to do it in the right way," said Mr Cameron.