US President Barack Obama and wife Michelle have been greeted in Downing Street by Gordon Brown and wife Sarah, reported BBC.
President Obama and the UK prime minister will hold talks over breakfast as efforts continue to ensure agreement is reached at the G20 summit.
Both Mr Brown and President Obama will also hold a series of meetings with other world leaders during the day.
The president will also meet Tory leader David Cameron and he is due to meet the Queen at Buckingham Palace.
World leaders are gathering in London to discuss ways to resolve the worst financial crisis since the 1930s. The summit takes place amid tight security and police warnings of "unprecedented" levels of protest.
Workers in London's financial district have been told to dress down and stay at home to avoid provoking demonstrators.
President Barack Obama arrived in London on Tuesday evening on his first visit to Europe since taking office, with hopes high that he can forge a new global deal.
The two-day summit officially begins on Wednesday evening as the leaders from the G20, which groups the world's most powerful economies, attend a reception with the Queen at Buckingham Palace.
But detailed bargaining will take place at a series of face-to-face meetings earlier in the day, starting with the breakfast meeting between Prime Minister Gordon Brown and President Obama.
The G20 summit will be crucial for Gordon Brown, whose reputation for economic competence has been battered by the financial crisis.
He has sought to lead the drive for a global approach to the crisis and the meeting is an opportunity to restore his image as a competent economic manager.
But it is unlikely to be smooth sailing. French Finance Minister Christine Lagarde has already said France would walk out of the summit if its demands for stricter rules are not met.
