British Prime Minister Boris Johnson was set Tuesday to announce his plans for the HS2 high-speed railway, with reports suggesting he will stick with the long-running project despite soaring costs.

HS2, which would quicken rail journeys between London in southeast England and cities to the north of the capital, has been dogged by controversy owing to projected spiralling costs and damage to wildlife.

It is forecast that the project, aimed also at cutting congestion on existing railway lines linking major UK cities, could cost more than ?100 billion ($129 billion, 118 billion euros) -- double an official 2015 estimate.

Media reports suggest Johnson could green-light the first phase of HS2 linking London with Britain's second-biggest city Birmingham in the English Midlands, but make changes to the second stage.

The second phase is for trains to travel further north to Manchester and Leeds, while the start of the line would be Britain's first new railway north of London in 150 years.

Following a convincing general election victory for his Conservative party in December, Johnson has pledged huge improvements to rail infrastructure across northern England as part of the government's bid to rebalance Britain's London-centric economy.

But by delaying or even scrapping the second phase of HS2, Johnson risks harming his wish to keep on board voters in the north who contributed heavily to the Conservatives' poll win.

The previous government last year noted that HS2 costs had soared owing to the complexity of building in densely populated cities and challenging ground conditions.

At the same time, it was revealed that rather than opening in 2026, the first phase of HS2 would not operate until between 2028 and 2031. Preparatory costs are said to have already hit ?8.0 billion.

It is forecast that the HS2 project could cost more than £100 billion  -- double an official 2015 estimate
It is forecast that the HS2 project could cost more than £100 billion -- double an official 2015 estimate AFP / Paul ELLIS

HS2 would be Britain's second high-speed rail project after HS1, or the line linking London with the Channel Tunnel that goes on to connect the UK with France.

Johnson meanwhile is awaiting the conclusions of a passenger-focused review of the UK's entire rail sector described by the Conservatives as "the most significant" since the Tories privatised British Rail in the mid-1990s.

The "root and branch" review of the rail sector, including improvements to freight travel, is chaired by Keith Williams, a former chief executive of British Airways.

Governments are coming under pressure to promote cleaner travel amid increasing public pressure to tackle climate change by slashing carbon emissions.

As part of a wider plan to promote greener transport, the German government last month agreed to pump 62 billion euros ($68 billion, ?53 billion) into its rail network.

Beyond the massive sum stumped up by the state, German rail operator Deutsche Bahn will plough another 24 billion euros into the renewal programme.

Getting more people to switch from cars and planes to trains is crucial to a plan aimed at helping Germany ensure its 2030 emissions are 55 percent lower than 1990 levels.

As part of the German climate package, train fares will go down while higher taxes will make air travel more costly.