Obama, who was feted on Tuesday by Britain's royal family, will hold talks with Prime Minister David Cameron at 10 Downing Street, and make a major speech to both houses of parliament on US-Europe relations.
The address, coming a day before the president heads to the G8 summit of developed nations in France, will be closely watched in European capitals, where some key players have felt the Obama White House has neglected transatlantic ties in favor of emerging regions like Asia.
It will also take place after the turmoil in North Africa and the Arab world, and the NATO assault on Moamer Kadhafi's forces in Libya, cast new scrutiny on Europe's handling of events in its own geographical backyard.
"What (Obama) will again underscore is both the essential nature of the US-UK alliance as well as the broader transatlantic alliance to global security and prosperity," said Ben Rhodes, a deputy US national security advisor.
"I think he'll speak to the fact that we've obviously come through a very difficult decade, but in some respects we're turning a corner," he said.
Rhodes noted that Obama was on the way to fulfilling his 2008 campaign pledge to withdraw all US troops from Iraq this year, and was on the cusp of beginning limited security transitions to Afghanistan's government.
But US officials also seem keen to rebrand the transatlantic alliance for a new age when the Cold War fears that bound western nations are only a memory, and will promote US-Europe ties as a catalyst for action on global security.
The lofty rhetoric though may not obscure apparent differences of emphasis on showdowns like the NATO campaign in Libya, where Britain and France are taking the military lead, backed by a more reluctant United States.
British, French and American officials insist the campaign against Kadhafi forces is not getting bogged down, but the longer the conflict goes on, the more pressure rises on Obama from his allies for a more robust US role.
The US president is seeking support after laying out a plan last week for Egypt and Tunisia to receive several billion dollars, in an effort designed to push the former autocracies towards genuine democracy.
Obama, who will hold a press conference with Cameron on Wednesday, basked in the ceremonial splendour of a state visit Tuesday laid on by Queen Elizabeth II, who has met every US president but one since 1950s leader Dwight Eisenhower.
A 41-gun salute boomed out over London and Buckingham Palace choreographed a white tie state dinner at which Obama and the 85-year-old British monarch swapped glowing compliments.
Prince William and his tanned bride Catherine meanwhile added a twist of glamour, meeting the US first couple just after returning from the Seychelles honeymoon that capped their fairytale wedding last month.
The queen said in her dinner toast that Obama's visit recalled "our shared history, our common language and our strong intellectual and cultural links."
"Your country twice came to the rescue of the free and democratic world when it was facing military disaster," she told Obama, in a reference to the two world wars.
Obama concluded his toast with a quote from Shakespeare's Richard III.
"To her Majesty the Queen, to the vitality of the special relationship between our peoples and in the words of Shakespeare, 'to this blessed plot, this earth, this realm, this England.'"
|US President Barack Obama high-fives with British Prime Minister David Cameron as they play table tennis with students of the Globe Academy school in London|
And breaking free from starchy convention, he drew a regal smile as he told the queen his daughters Malia and Sasha "adored" her.
There was one hiccup when the string orchestra of the Scots Guards began playing the British national anthem before Obama had finished his speech, resulting in a breach in protocol as the president continued to talk over the music.
However, the queen did not appear to be offended and thanked the leader for his "very kind" words.
Earlier, in a slice of history, at Westminster Abbey, America's first black president passed beneath a statue of civil rights leader Martin Luther King, installed above the ancient abbey's Great West Door in 1998.
The president then laid a wreath on the tomb of the unknown warrior, an unidentified soldier of the Great War who has come to represent Britain's entire war dead.