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Sadiq Khan’s controversial plan to move City Hall to east London could be delayed by unexpected snags at the new building.
Construction teams converting the Crystal have discovered flood damage to the floor.
Consultants have also ordered additional security measures because of concerns about a nearby public road.
The Greater London Authority has drawn up contingency plans to hold London Assembly meetings at the Barbican Centre or South Bank Centre.
The mayor, Assembly members and GLA are due to leave City Hall by Christmas after Mr Khan rejected a new lease.
He claimed the move to the Crystal, a former exhibition centre in the Royal Docks, would save £61m over five years.
But the cost of the switch has already risen from £8m to almost £14m.
"There's always snags and issues that come up in any major construction, in any infrastructure project. The good news it was spotted early. We're going to make sure we sort out the floor, sort out the problems in relation to the flooding. I can guarantee it will make savings when we move to the Crystal," said Sadiq Khan, Mayor of London.
City Hall, on the south bank of the Thames close to Tower Bridge was designed by Norman Foster and opened by the Queen in July 2002.
Some assembly members were angered by the decision to abandon an iconic central London HQ in favour of a new home downstream.
"Ultimately this was a decision by the Mayor of London to give up our purpose-built building in central London to move out to the east to a very small building that isn't fit for purpose. And now we're seeing more and more costs around this. Ultimately there's going to be hardly any savings out of this move," said Caroline Pidgeon, chair of the London Assembly Oversight committee.