Construction of the world’s most expensive railway ever built is now “formally underway” in the UK, as contractors working on HS2 move from preparatory works and design to begin building the controversial rail link.

Prime minister Boris Johnson, who endorsed the plan earlier this year after he said “in a hole the size of HS2, the only thing to do is keep digging”, is due to attend an as yet undisclosed site on Friday where he will stage a “shovels in the ground moment”.

The government gave the £106bn project the green light for construction in April at the height of lockdown, saying it could not be delayed any longer.

Mr Johnson said the north-south rail link, which has already run tens of billions of pounds over budget and several years behind schedule, would “fire up economic growth and help to rebalance opportunity”.

“HS2 is at the heart of our plans to build back better – and with construction now formally underway, it’s set to create around 22,000 new jobs,” the prime minister said.

“As the spine of our country’s transport network, the project will be vital in boosting connectivity between our towns and cities.”

The original budget for the project was £32.7bn when it was first unveiled in 2012. But costs have steadily risen and a government-commissioned review warned last year that the final bill for HS2 could reach £106bn at 2019 prices.

It means the railway will ultimately cost over £307m per mile for the 345 miles of high speed line.

Critics have said there are better ways to spend over £100bn on rail infrastructure, and the project continues to face opposition on environmental grounds.

The first phase of the project will instal a high-speed rail link between London and Birmingham, and then on to Manchester and Leeds in the second phase.

The complete HS2 network is unlikely to be ready until 2040.

It is the largest infrastructure project in Europe, and involves the construction of more than 300 bridges and 70 viaducts for the first phase alone.

Construction will begin with the biggest engineering challenges – such as the stations and tunnels – followed by the main viaducts and bridges.

Most activity this year will be focused on HS2’s city centre stations and major construction compounds such as in Old Oak Common, west London and Calvert, Buckinghamshire.

A report by the organisation released in January warned HS2 will damage or destroy 108 ancient woodlands, five internationally protected wildlife sites, and 693 local wildlife areas.

Wetlands, wildflower meadows and historic wood pasture are all among those which will either be lost or significantly damaged by the new line, according to the study.

In March, the RSPB also criticised the impact of the project, and said after the organisation’s suggestions on how to preserve biodiversity were ignored, that “the scheme has descended into an example of worst, rather than best, practice.”

The RSPB said it was not only disappointed with the government’s decision to proceed with HS2, “but also with the wild claims that it will be an environmentally leading scheme”.

At the time HS2 said: “HS2 Ltd takes the environmental cost of construction very seriously. That is why before any works take place on site, surveys are conducted by qualified ecologists to ensure we meet legal tests, and we have tailored mitigation plans in place for protected species.

“We’re also delivering an unprecedented programme of tree planting and habitat creation alongside the new railway – with seven million new trees and shrubs set to be planted between London and Birmingham alone.”​

Transport Secretary Grant Shapps claimed the formal start to construction on Friday “marks a major milestone in this government’s ambitions to build back better from Covid-19”.

He said: “Shovels in the ground to deliver this new railway means thousands of jobs building the future of our country’s infrastructure.

“This fantastic moment is what leaders across the north and Midlands have called for – action to level-up our country by boosting capacity on our railways, improving connections between our regions, and spreading prosperity.”

HS2 Ltd chief executive Mark Thurston said: “This is a hugely exciting moment in the progress of HS2. After 10 years of development and preparatory work, today we can formally announce the start of full construction, unlocking thousands of jobs and supply chain opportunities across the project.

“We are already seeing the benefits that building HS2 is bringing to the UK economy in the short term, but it’s important to emphasise how transformative the railway will be for our country when operational.

“With the start of construction, the reality of high-speed journeys joining up Britain’s biggest cities in the north and Midlands and using that connectivity to help level up the country has just moved a step closer.”

Additional reporting by PA