The ticking Blitz bomb that threatens east London

The 2,000lb device, believed to contain 1,000lb of high explosives, has been encased in a sand and wood "igloo" to absorb any blast if it goes off.

Tonight Royal Engineers plan to cut through the metal casing of the 60-yearold-device, which measures 5ft by 2ft. They will "steam" the explosive so it becomes a safe liquid.

The Second World War bomb was dredged from the banks of the Lea near Three Mills Island in Bromley-by-Bow near the 2012 Olympics site.

Disposal experts have already disabled its fuse after it began to tick. At one point they asked police to consider evacuating up to 1.7 square kilometres - an area which includes thousands of people, schools, key parts of the rail and Tube network and much of the Olympic site. Putting aside standard procedures, one bomb disposal officer was sent back to the device to deal with its fuse.

He poured a salt solution on to it to "freeze it" and then deployed a powerful magnet in case a timer had also been set off.

The officer, Captain Si Oates, would normally be under orders only to approach a live device twice.

"This would have been an enormous blast with a devastating impact on the surrounding area from the point of impact. We are talking about a ton of explosive here," a military source said. A 200-metre exclusion zone is now in effect around the device.

The plan to make the bomb safe will bring travel chaos from 10pm tonight as nearby Tube and rail lines are closed to avoid passengers being put in danger.

The District and Hammersmith and City lines between Bromley-by-Bow and West Ham, the C2C line to and from Fenchurch Street and a string of local bus services will be cancelled or diverted.

Commuters have been advised to make alternative travel plans until it is moved.

The bomb was discovered on Monday. Police said around six residents in the area were evacuated as well as a number of businesses in the 200-metre exclusion zone.

Borough Commander Simon O'Brien said: "It has been an interesting few days for us, which has been happening in the background while London eats and sleeps. Our Royal Engineer colleagues have been extremely brave.

"From what I have been told you do not go back to a live device more than twice. The military originally wanted a kilometre and half exclusion zone and that gives you an idea of the area that could have been affected."