The light aircraft flown by Mr Kennedy is believed to have crashed into the sea off the eastern state of Massachusetts late on Friday.

Rear Admiral Richard Larrabee of the US Coast Guard said: "We are going to shift from our focus on search and rescue to search and recovery."

Stephen Sackur reports: "The wait for news has been painful and increasingly hopeless"

The admiral said the decision had been taken after evaluating the probability of survival after so many hours in waters of the Atlantic.

"I have spent some very painful moments with the families tonight," Admiral Larrabee added.

Clinton: 'We offer our prayers and those of all Americans'

"They have all been very gracious and very appreciative ... and they thank all of the people who were involved."

Mr Kennedy and his wife Carolyn had been travelling to the family home at Hyannisport for the wedding of a cousin Rory - the daughter of the late Senator Robert F Kennedy - when the aircraft disappeared.

The wedding, due to take place on Saturday before 275 guests, has been postponed.

Clinton's prayers

In an address to the nation, President Clinton called on Americans to pray for Mr Kennedy, his wife and her sister Lauren Bessette, who was also onboard the aircraft.

In his address, President Clinton said: "As the search continues, I want to express our family's support and offer our prayers and those of all Americans for John Kennedy Junior, his wife, Carolyn, her sister, Lauren, and to their fine families."

He was speaking at the White House after a weekend break at the presidential retreat in Camp David, Maryland.

An investigation into a "very brief" electronic transmission, or "ping", from what was thought to be an emergency locator beacon, was stopped after the Coast Guard discovered it was a false alarm.

A research vessel from the National Oceanic and Atmospheric Administration was diverted from its search duties to investigate the transmission.

Mark Devenport reports: "The coastguard have no reason to be optimistic"

Officials said divers would begin searching possible target areas in water 60ft to 80ft deep on Monday.

Small pieces of wreckage and luggage from the aircraft were found on a Massachusetts beach.

Nationwide mourning

The disappearance of Mr Kennedy - son of the assassinated US president - has caused a deep sense of shock across the United States.

Admiral Larrabee: 'It was difficult to share this information with the family'

Prayers for the Kennedys have been said at churches throughout the US, and a special mass was held at the family compound at Hyannisport.

In New York a steady stream of people has been taking flowers and candles to his home in downtown Manhattan.

Mr Kennedy, 38, a law graduate, was born just two months before his father's inauguration as the 35th US president.

He never entered politics, choosing instead to observe via the pages of his glossy political magazine George - subtitled "not just politics as usual".

People magazine once labelled him "the sexiest man alive".

The BBC's Richard Quest: "Search and rescue has shifted to search and recovery"

As television networks suspended their scheduled programming to focus on the search, US Vice President Al Gore cancelled campaign appearances, calling for a moment of prayer for the missing people.

Radar footage showed the plane - a single-engine Piper Saratoga II HP - was making a normal descent towards the island when it plunged from 2,200ft to 1,300ft in just 12 seconds, before disappearing from radar screens.

• What happened to JFK Jnr's plane?
• The Kennedys: A family cursed
• JFK Junior: Born into fame and tragedy
• Picture gallery: Life in the limelight
• Kennedy makes world headlines
• New York's sorrow
• Carolyn: Camelot's Queen