World leaders have been congratulating Conservative leader David Cameron on his appointment as prime minister.
US president Barack Obama phoned Mr Cameron to congratulate him within an hour of him taking office.
The president said he told Mr Cameron he remained committed to a "special relationship" between their countries and invited him to visit Washington this summer.
"The United States has no closer friend and ally than the United Kingdom, and I reiterated my deep and personal commitment to the special relationship between our two countries - a bond that has endured for generations and across party lines, and that is essential to the security and prosperity of our two countries and the world," Obama said in a statement.
Chancellor Merkel phoned to offer her congratulations and invite Mr Cameron to Berlin at the earliest opportunity, Downing Street said. “They briefly discussed the world economy and the common European agenda," the statement said.
French President Nicolos Sarkozy offered his "hearty congratulations” to Mr Cameron, a statement from the Elysee Palace said. “He (the president) offers his best wishes for success ... And he hopes to work with him to reinforce the tight cooperation and the exceptional ties connecting France and the United Kingdom."
Canadian prime minister Stephen Harper said Britain and Canada "share a deep and enduring relationship, forged in good times and bad by hundreds of years of shared history, values and tradition”.
"We are united by the cause of freedom, democracy, human rights and the rule of law in both our respective countries and throughout the world. I am certain that the bonds between our two countries will deepen over the coming years."
Last night, European Commission Jose Manuel Barroso underlined the need for common responses to economic challenges in a congratulatory note to Mr Cameron.
"On behalf of the European Commission, I would like to offer you my warmest congratulations on your election as Prime Minister of the United Kingdom," Mr Barroso wrote. "Many of the challenges ahead - delivering economic recovery, fighting global poverty, tackling climate change, ensuring energy security - are common across the European Union and require a common response."
He said he looked forward to working with Cameron on these and other issues, such as boosting the internal market and promoting "smarter regulation", more transparency and greater accountability in the European Union.