Prime Minister Gordon Brown will miss a nuclear summit hosted by U.S. President Barack Obama in Washington next week as he campaigns for the May 6 parliamentary election, his spokesman said on Tuesday.

Brown has asked Foreign Secretary David Miliband to represent him at the April 12-13 nuclear security summit.

"He will go with the prime minister's full authority," Brown's spokesman said.

Brown earlier launched a month-long election campaign likely to be dominated by the economy and shaping up as the most unpredictable contest since 1992.

The Conservatives are ahead of Labour in opinion polls.

Brown let Obama know about his plans in a regular phone call with the U.S. president that also covered Afghanistan, Iran and the global economic recovery, according to White House National Security Council spokesman Mike Hammer.

Chinese President Hu Jintao is among world leaders who will attend the Washington summit, which will focus on securing nuclear material to prevent it falling into the wrong hands.

The Obama administration unveiled a new policy on Tuesday restricting U.S. use of nuclear weapons but warned Iran and North Korea that they remain potential targets.