Activists who dug a tunnel near Euston station in protest against the £106bn HS2 rail project are at "risk of drowning", officials have warned.
Bailiffs are still evicting people from the Euston Square Gardens camp after protesters dug a tunnel they claim is 100ft (30m) long.
At least four people are inside the tunnel, which took months to dig.
The National Eviction Team said there was a danger of it collapsing and putting lives in danger.
Protesters have exposed themselves to "significant risk" by being in the "crudely dug tunnel", the High Court Enforcement Group (HCEG) added.
A spokeswoman said HCEG was aware those inside the tunnel had already experienced a collapse and water ingress.
She added: "The unlawful activists appear to have put themselves in danger of a further tunnel collapse, and potentially of intercepting nearby gas and water pipes, leading to risks of suffocation, flooding and drowning."
Since August, HS2 Rebellion members have been living in tree houses and tents at a camp near Euston station.
Activists say the tunnel - codenamed "Kelvin" - was dug under Euston Square Gardens as their "best defence" against being evicted.
Specialist equipment is now being used to monitor air conditions to ensure the protesters underground are not exposed to gas or water.
Because of the hazards, the HCEG spokeswoman warned it would take time before the demonstrators were removed.
She added: "Many of the activist occupiers of the land are known to us from previous protest activity, and we can expect little co-operation from some of these individuals."
Transport Secretary Grant Shapps described the protesters' actions as "reckless, irresponsible and deeply concerning".
"These illegal occupations not only endanger the lives of themselves and those working to ensure their safety, but put an unnecessary strain on the skilled officers and paramedics of our emergency services," he said.
The Met Police said five more people have been arrested at the campsite.
Bailiffs have been using cherry pickers to remove activists who have perched themselves in tree houses.
HS2 is set to link London, Birmingham, Manchester and Leeds. It is hoped the 20-year project will reduce rail passenger overcrowding and help to rebalance the UK's economy.
The campaign group alleges HS2 is the "most expensive, wasteful and destructive project in UK history" and that it is "set to destroy or irreparably damage 108 ancient woodlands and 693 wildlife sites".
However, HS2 bosses said seven million trees would be planted during phase one of the project and that much ancient woodland would "remain intact".