Two boys aged 13 and 14 are facing a police rap after trespassing and starting a fire at the old Abingdon Street Post Office.
Fire crews had to clamber through windows at the famous building to put out the blaze at 4.20pm on Boxing Day.
Witnesses reported a passer-by narrowly escape injury when a glass pane from the building, which was built in 1910, crashed to the pavement below.
When police were alerted, two youths were found inside the property.
They were detained and will be interviewed by police at a later date.
Watch manager Warren Topp, from Forest Gate Fire Station, said four crews attended and they were faced with a difficult job.
He said: “It was an extremely dangerous site as there were parts of the floor missing and holes everywhere. It’s all being ripped out as part of being redeveloped. But if a fire took hold it could still cause a big issue.
“We managed to get a 13.5m ladder up to the first floor from the front of the building and then gained access to the second floor with a triple extension ladder to be able to investigate. There was a fire but it had burnt itself out.”
A spokesman for Lancashire Police said: “Two boys aged 13 and 14 were detained at the scene and will be voluntarily interviewed at a later date.”
The site is currently derelict awaiting redevelopment for a leisure and shopping hub called the ‘Red Box Quarter’.
The post office building was built in 1910 and was used until 2007 when the counter service moved to WH Smiths.
Planning permission includes the removal of the former post office counter and its re-use to form a central kiosk within the main hall on the ground floor.
The original proposal for a 50-bedroom hotel was shelved in favour of investment in retail.