The government has given formal approval for construction work on the HS2 rail project to begin despite lockdown measures.
Construction firms involved in phase one of the high-speed rail project will need to follow social distancing rules.
HS2 minister Andrew Stephenson said: "We cannot delay work on our long-term plan to level up the country."
Prime Minister Boris Johnson approved the decision to build the rail link in February after a review into its cost.
Matthew Kilcoyne, deputy director of the free-market Adam Smith Institute, called the government's announcement "tone-deaf" in the light of the coronavirus pandemic.
Mr Kilcoyne said: "We've got an economic crisis that's going to cost taxpayers billions. We can't afford vanity projects like HS2.
"We need to get back on to a sustainable financial footing."
The government's official report previously warned that the project could cost more than £100bn and be up to five years behind schedule.
On Tuesday Chancellor Rishi Sunak warned that the coronavirus pandemic "will have serious implications for the UK economy".
He spoke after the Office for Budget Responsibility (OBR) estimated that a three-month lockdown would hit GDP and push up the UK's borrowing bill to an estimated £273bn this financial year.
HS2 minister Andrew Stephenson said: "This next step provides thousands of construction workers and businesses across the country with certainty at a time when they need it."
A notice to proceed has been given to four joint ventures, which will start work immediately, according to a statement by the Department for Transport (DfT).
The announcement was welcomed by the boss of the Construction Industry Council, Graham Watts. "The notice to proceed with HS2 is welcome at this time, particularly for the benefit of the economy," he said.
"When the current crisis is over, planned recovery is vital and major infrastructural work such as HS2 and from Highways England, together with a recovery in housebuilding, is a key instrument for kickstarting the wider economy."
Companies with HS2 contracts include Costain, Balfour Beatty and Skanska Construction UK.
Mark Thurston, chief executive of HS2 Ltd, said: "The issuing of notice to proceed today ensures that our contractors and their supply chains have the confidence that they can commit to building HS2, generating thousands of skilled jobs across the country as we recover from the pandemic."
Safety of the workforce
Construction workers on-site will need to observe Public Health England's advice on social distancing during the Covid-19 outbreak.
The GMB union, which represents HS2 construction workers, said that the safety of the workforce "must be the overriding priority".
Eamon O'Hearn, its national officer, said that construction should be "conditional on rigorous observation of social distancing, provision of personal protective equipment where required", as well as individual risk assessments for vulnerable workers.
HS2 is set to link London, Birmingham, Manchester and Leeds. It is hoped the project will reduce passenger overcrowding and help rebalance the UK's economy through investment in transport links outside London.
HS2 minister Mr Stephenson added: "HS2 will be the spine of the country's transport network, boosting capacity and connectivity, while also rebalancing opportunity fairly across our towns and cities."