Tony Blair has become the first leading British politician to meet Barack Obama since he was sworn in as US President.
The former prime minister was welcomed as a "very good friend" by Mr Obama, as he gave a speech at the traditional National Prayer Breakfast.
He advised the new president: "You don't need cheerleaders but partners, not spectators but supporters."
Mr Obama praised Mr Blair as "somebody who did it first and perhaps did it better than I will do".
In his speech, Mr Blair had some words of advice for the new president, telling him: "The truest friends are those still around when the going gets toughest."
Mr Blair, who converted to Catholicism after leaving office, warned that the "public eye is not always the most congenial".
"Should it ever be tested, I hope your faith can sustain you and your family," he said.
After the address Mr Obama said: "I want to thank my good friend Tony Blair for coming today. He has been an example to so many people around the world of what dedicated leadership can accomplish."
Shortly after Mr Obama's inauguration, Downing Street played down claims there was an unofficial race between European leaders to be the first to meet the new president.
But Gordon Brown was the first European leader to receive a post-inaugural phone call from Mr Obama.
The president is due to visit the UK in April for a meeting of G20 leaders focusing on the economic downturn.
During his speech, Mr Blair joked about the differences between faith and politics in the US and the UK.
He told the audience of the "complete consternation" when, as PM, he wanted to end an address to the country with "God bless the British people".
"The system was aghast ... As I sat trying to defend my words a senior civil servant said with utter disdain, 'Really prime minister, this is not America you know'," he recalled.
He ended his keynote speech: "By the way, God bless you all."