Police, fire and ambulance services attended the Hammersmith Town Hall site in King Street after the incident at around 5.41pm on Wednesday.
A spokesman for the London air ambulance service said: “Two men were found with potentially life threatening injuries and were taken to a central London hospital. The Health and Safety Executive has been informed.”
Hammersmith and Fulham Council confirmed that the men were working on the new Civic Campus build.
“We are sorry to report there has been an accident on the Civic Campus construction site. The site is currently not under H&F’s control (it is managed and operated by the construction company) so it is hard to glean exact information.
“Our thoughts are with those injured, their colleagues, friends and family. Further updates to follow.”
Ardmore’s £155m job involves refurbishing and extending the existing Grade II-listed Town Hall building to provide over 15,000 sqm of office and public space alongside 204 new homes and 10,000sqm of commercial space.