An inquiry should be set up after concrete collapsed at the entrance of a Crossrail tunnel, the Rail, Maritime and Transport union has said.
No-one was injured in the incident, which happened on 9 March at Royal Oak in west London.
A spokesman for Crossrail confirmed an investigation was under way.
But the union described it as a "major incident" and called for a "full Health and Safety Executive investigation".
"Clearly this was a major incident which could have had serious repercussions and rather than the attempt to sweep it under the carpet there should be a full Health and Safety Executive investigation into the structural failure that caused this tunnel collapse to ensure that lessons are learnt and it doesn't happen again," RMT leader Bob Crow said.
The Crossrail spokesman said the construction project, which has been operating for three years, "has an excellent safety record".
"On Friday 9 March, a small amount of temporary concrete came loose and broke free from the face of the tunnel portal at Royal Oak," he said.
"No injuries resulted and repairs were quickly undertaken."
The £16bn project is to due to be completed by 2018, and will pass through 37 stations on a 73-mile (118km) route, from Maidenhead and Heathrow in the west, to Shenfield and Abbey Wood in the east.
- 15 March 2012