Sky Sports News HQ can reveal that accountancy firm Moore Stephens has been appointed to investigate the rising costs of the transformation of the London Olympic Stadium into the home venue for West Ham.
The firm has been tasked with looking into the increased costs by Mayor of London Sadiq Khan, who ordered the investigation late last year.
Transformation costs have reached £323m, more than £50m over the estimates to convert it into a 60,000 multi-purpose venue.
Mayor Khan announced his investigation in November after Sky News learned that the estimated annual cost of moving "retractable" seats, installed to improve the view for football, had risen to £8m, and that moving the seats back and forth for summer events including athletics could take a month.
West Ham have a 99-year lease on the venue and pay an annual rent of £2.5m - they also contributed £15m towards the conversion costs.
Draft findings are due to be presented to the Greater London Assembly by June 2017 and a public report published in the summer.