An anti-HS2 campaigner has been arrested after voluntarily leaving tunnels dug in secret by activists near Euston station in London.

A group of at least six people has spent more than a week beneath Euston Square Gardens as part of a protest over the impact of the planned high speed railway.

A spokesman from HS2 Ltd said on Friday night: “One of the illegal trespassers has voluntarily left the underground tunnel this evening, and we urge the others to follow.

“On-site paramedics offered immediate medical assistance, which was refused.

“The trespasser was then arrested by the Metropolitan Police and taken to a London hospital as a precaution.”

The spokesman said the safety of protesters, HS2 staff and emergency services personnel was of “paramount importance” and that it was doing “all we can to end this illegal action safely”.

He reiterated calls for activists to exit the tunnels “as soon as possible”.

Roc Sandford, father of another tunnel activist Lazer Sandford, who bailiffs were trying to extract from a “lock on” at the bottom of a down shaft, told the PA news agency that his son had “escaped” from them and was now “resting in the depths of one of the tunnels”.

Bailiffs had been working for hours to remove the demonstrator who had attached himself to the steel and concrete device.

HS2 Rebellion, an alliance of groups and individuals campaigning against the scheme, claimed that by Saturday morning the protest in Euston will become the “longest UK protest tunnel occupation in two decades”.

Mr Sandford, who is also father of 18-year-old Blue Sandford, another protester in the tunnels, earlier said he was “scared and distressed” as their parent but was “right behind them” and understood “exactly why they know they have to do this”.

Emailing the PA news agency from his home on the Hebridean island of Gometra, he said: “HS2 needs to be put out of its misery.”

Mr Sandford said he speaks with his children “regularly”, describing them as “completely resilient, upbeat and brave”.

He said Blue and Lazer had helped him on previous protests against Scottish salmon farming, which “had a big impact on them both”.

The family have also protested with the environmental campaign movement Extinction Rebellion.

Commenting on the tunnellers he said: “I am in awe of all of those incredibly brave spirits down there, not just my children, but all of them.”

Confined space rescue specialist Peter Faulding said overall the eviction could last up to “six weeks” and end up costing “millions of pounds”.

Mr Faulding, founder of Specialist Group International and an expert witness for the protesters’ legal team, criticised the handling of what he alleged was a “rushed” operation.

Speaking to PA, he outlined the scale of the challenge facing bailiffs: “They’ve got to dig that tunnel, they’ve got to follow that tunnel and they’ve got to dig it it out to at least well over a metre and shore every inch of it up to be safe.

“This is unstable ground.

“So you’re looking at a very slow operation.”

Mr Faulding, who worked on a number of tunnel protest evictions in the 1990s, said the situation in Euston was a “public relations nightmare” for HS2, that “could have been done so much better”.

He warned eviction was going to become “extremely difficult and extremely dangerous”, with the potential for someone to get “badly injured”.

Mr Faulding said air monitors were “vital” in the tunnels to avoid the risk of hypoxia and “hardwire” communications were needed to maintain contact in case there was a tunnel collapse.

HS2 Rebellion has called on the Government to scrap the “expensive, unpopular and destructive” railway scheme and claims plans will see Euston Square Gardens built over with a temporary taxi rank before being sold to developers.

Activists in the tunnels include veteran environmental campaigner Swampy, real name Daniel Hooper, and his son Rory.

Earlier this week a High Court judge rejected a legal bid to halt efforts to evict them.

Mr Justice Robin Knowles refused an application brought by Dr Larch Maxey for an injunction requiring HS2 and others to cease operations.