HS2 protests against the high-speed rail project has cost up to £80 million, the government has said.
Environmental activists have caused disruption at several sites on Phase One of the line between London and Birmingham.
An expensive month-long operation was required to remove people from a network of tunnels in London’s Euston Square Gardens earlier this year.
In a statement to Parliament, HS2 minister Andrew Stephenson said: "With regard to protester activity, which HS2 Ltd estimates has cost the project up to £80 million, the Government is making sure that HS2 Ltd, its supply chain, emergency services and wider government have a co-ordinated response to illegal protest.
"Regrettably, some protesters have turned to violent and aggressive behaviour, particularly against HS2 Ltd’s supply chain."
He added: "The Government is taking steps to ensure that illegal protestor activity is properly dealt with and that safety risks are minimised."
Mr Stephenson said HS2 is facing "cost pressures of around £1.3 billion", up from the figure of £800 million given in his previous update in March.
Some £600 million is due to the "slower than expected mobilisation" of construction contracts partly due to delays with completing preparatory work, approving designs and securing planning consents.
A further £400 million is due to difficulties with redeveloping Euston station.
Mr Stephenson explained the costs may need to be covered by contingency funds contained within HS2’s existing £44.6 billion budget (at 2019 prices) for Phase One agreed last year.
HS2 Ltd has also identified more than £300 million in savings and "continues to focus on realising further efficiencies and opportunities to reduce the cost of Phase One", the minister added.
He also confirmed the number of HS2 platforms at Euston will be scaled back from 11 to 10, despite concerns about the potential impact on punctuality of services.
Mr Stephenson wrote: "This will provide a more efficient design and delivery strategy and play a significant role in mitigating the affordability pressures recently identified.
"Moving to this revised HS2 Euston station design maintains the station infrastructure capacity to run 17 trains per hour, as set out in the Phase One full business case."
Some £15.3 billion (in actual prices) has already been spent on the project, including through land and property acquisition.
The Government-commissioned Oakervee Review warned in 2018 the final bill for HS2 could reach £106 billion (at 2019 prices).
Despite it running tens of billions of pounds over its initial budget and several years behind schedule, Boris Johnson gave HS2 the green light in February 2020.
A Department for Transport spokeswoman said: "HS2 Phase One remains within its schedule and budget. Delivering a major project of the scale and ambition of HS2 will always be challenging but, as today’s parliamentary report sets out, current cost pressures, if they materialise, will be covered within our existing budget.
"This project is at the heart of our plans to build back better, supporting more than 20,000 jobs.
"We are committed to maintaining a tight control of costs and HS2 Ltd continues to identify areas across the project where savings and efficiencies can be made."