The safety of its workforce and supply chain must be the overriding priority.
Ministers have given the go ahead for the HS2 project to enter the construction phase.
HS2 Ltd, the Government-owned company developing the high speed railway, was given approval to issue a notice to proceed to firms contracted to design and construct bridges, tunnels, embankments and viaducts for phase one between London and Birmingham.
Prime Minister Boris Johnson gave the green light for HS2 in February despite it running tens of billions of pounds over budget and several years behind schedule.
But many building sites, such as those developing Crossrail, are closed due to the Covid-19 lockdown, while some are open with fewer workers than normal to comply with social distancing guidelines.
A campaign was launched to close all building sites because of the threat to workers.
Eamon O'Hearn, national officer of the GMB union, called for rigorous standards to protect workers.
He said: "GMB wants to protect workers' jobs, health and pay. The safety of its workforce and supply chain must be the overriding priority."
Andrew Stephenson, the HS2 minister, said the government's main priority is combating the spread of coronavirus but immediately undermined this point by adding: "We cannot delay work on our long-term plan to level up the country".
The government-commissioned Oakervee Review warned that the final bill could reach £106 billion at 2019 prices.
The UK's construction industry employs more than two million workers and produces about six percent of the country's total economic output.
Four joint ventures were awarded main works civil contracts worth a total of £12 billion for phase one of HS2 in July 2017.
- SCS Railways (Skanska Construction UK, Costain and Strabag)
- Align joint venture (Bouygues Travaux Publics, Sir Robert McAlpine and VolkerFitzpatrick)
- EKFB joint venture (Eiffage Genie Civil SA, Kier Infrastructure and Overseas, Ferrovial Agroman and BAM Nuttall)
- BBV joint venture (Balfour Beatty Group, Vinci Construction Grands Projets, Vinci Construction UK and Vinci Construction Terrassement)
Shares in Costain, which is listed on the London Stock Exchange, jumped by around a third on the news that a notice to proceed has been issued.
Preparatory work at some HS2 sites remains paused following a review of what operations can be carried out in accordance with coronavirus guidance.
But HS2 Ltd said the four joint ventures will "start work immediately" on detailed designs, site preparation work and issuing sub-contracts.
The companies involved claim they are applying conditions such as ensuring a minimum clearance between all personnel of two metres, having facilities for regular hand washing and single occupancy of all vehicles unless agreed in advance.
It is not clear how agreement in advance for shared vehicles can actually prevent the spread of a highly contagious virus.
Phase one of HS2 was due to open in 2026, but it could be 10 years later before full services operate.
Passenger trains between Old Oak Common in west London and Birmingham Curzon Street are due to start running between 2029 and 2033, with the line extended to London Euston between 2031 and 2036.
The DfT and HS2 Ltd's estimate of the cost of phase one ranges from £35 billion to £45 billion.
Neil Lancefield is the PA transport correspondent.