The job was originally estimated at £100m last year when BAM was lined up as main contractor.
But BAM walked away from the deal last summer after failing to agree a price under a two-stage tender process.
Intu confirmed in a trading update this morning that “at intu Watford, we have completed the demolition of the outdated Charter Place precinct adjacent to the centre and have commenced the fixed cost building contract for the £180 million extension.”
Keltbray has completed the demolition work at the site ahead of main construction by Laing O’Rourke.