The body of their friend, Christopher Scott, aged 11, who was with them when they left for a bicycle ride on Sunday, was found on the marshes near Mablethorpe in the early hours of yesterday morning.
The two boys - 9-year-old Nathan Sawyer, and Ian Smye, who was 10 on Sunday - have not been seen since Sunday lunchtime when they left for a bike ride to celebrate Ian's birthday.
The last sighting of them was at Horse Shoe Point on the coast just a few miles from their homes in North Cotes, by a man who saw the figures of three children in the distance at around 1pm.
The alarm was raised on Sunday afternoon by the boys' families, and police were called after their bikes were discovered abandoned at Horse Shoe Point.
The area is known for high tides which can sweep in perilously fast, and it is feared that the boys were cut off.
Hundreds of people yesterday joined police and coastguards in their search for the two missing boys. The search was called off during the afternoon while the tide came in again, and resumed at 6pm. The in-shore lifeboat and helicopter crews from RAF Leconfield were keeping watch.
A coastguard spokesman said Sunday's tide, which measured 8 metres, was high. It usually varies between 6.2 metres and 8.8 metres. "It is a very dangerous area to be in when the tide is coming in," he said.
Parts of the marshes have posts driven into them with stages on which stranded walkers can take refuge, and signs throughout the area warn of the treacherous tides. Coastguards and locals are well aware of the dangers around that part of the mouth of the Humber, opposite Spurn Head spit.
When the tide is out, it is possible to walk two miles out on the sand but the danger is apparent when the flood tide comes in quickly, sometimes at faster than walking speed.
Someone caught unawares on the sand can be cut off as water surrounds them. Another hazard is falling into deep gullies running along the sand which fill up when the tide comes in.
Keith Vardy, the district controller of Humber Coastguard, said the boys might have fallen victim to the sea.
"The possibilities are that they could have been playing on the sand and were cut off by the tide coming in behind them or they may well have slipped into one of the gullies.
"We have not given up hope for them. The search will continue all day."
However, as darkness fell last night hopes were fading for the safety of the two boys.Reuse content