Mike Luedicke, who is co-ordinating the operation and is deputy commander of the Army’s UK bomb disposal unit, said they “very confident it was a classic Second World War German air drop bomb”.
“It’s called an SC2 50 – 250kg in weight, 139kg of high explosives,” he said.
He said a “large cave of sand bags” consisting of 250 tonnes of sand had been put around the site.
How the bomb measures up
SC 250 (Sprengbombe Cylindrisch 250)
164cm (65 in)
8 times the weight of an average labrador retriever (30kg/66lb)
11cm (4 in) shorter than the average man in England (175.3cm/5ft 9in)
West Midlands Ambulance Service
Major Luedicke said it had been a particularly “complex” case because the fuses were “on the underside and pressed into the earth and so our process of identification and diagnostics has been a real challenge”.
“We need to either magnetically or chemically freeze the fuses to essentially put a handbrake on them,” he said.
The A38(M) Aston Expressway, linking the city centre with the M6, has been closed since Monday morning.
Exit and entry slip roads at the M6 at junction six, Spaghetti Junction, remain closed.
London Midland rail services on the Cross City Line between Lichfield and Birmingham New Street have been suspended.
Delays of at least 30 minutes are being reported on the roads as motorists take alternative routes.
Radio WM reporter James Fanning said his journey from Aldridge to the city centre took nearly 2.5 hours, rather than the usual 45 minutes.
“It’s an absolute nightmare,” he said. “I must have take the worst route possible.”
A 500m cordon was set up after the discovery at about 09:45 BST on Monday and hundreds of homes and businesses evacuated.
About 80 people were forced to spend the night in a rest centre, the Red Cross said.
Paige Collins, of Aston, and her 10-month-old daughter, were among residents who spent the night at Alexander Stadium in Perry Barr.
“We’re in a dusty gym – I think it is normally used for martial arts. It is a bit draughty,” she said.
Ms Collins said it was mainly families there and she slept for about “an hour” on a mat and sheet after putting her daughter to sleep in her pushchair.
“I was trying to go into my house about 2:45pm [on Monday] and the police were at the bottom of my road saying everyone has to be evacuated,” she said.
Residents were told they could be there until about 18:00 BST, she said.
‘Humbled by bravery’
National Rail Enquiries said trains between the city and Lichfield Trent Valley were expected to be cancelled or amended until the end of the day and rail tickets would be accepted on buses.
Ch Supt Chris Johnson, of West Midlands Police, said he was “humbled by the bravery” of the bomb disposal experts who had been at the site overnight making the device safe.