Image copyright Siemens/ PA Image caption A proposed design for an HS2 train

The government is set to approve the controversial High Speed 2 (HS2) rail project, the BBC has learned.

An announcement on the route linking London to Birmingham, and then on to Manchester and Leeds, is expected by the prime minister on Tuesday.

Supporters of the plan say it will improve transport times, create jobs and rebalance the UK's economy.

But the rising cost of the project and accusations of mismanagement have provoked anger from critics.

HS2 - which is due to be completed by 2040 - is already over budget and behind schedule.

The government is expected to approve the whole line. However, it will try to make changes to the second phase of the project - the routes to Manchester and Leeds - to save money.

Adam Marshall, director general of the British Chambers of Commerce, told the BBC the train line would help cut overcrowding: "We need a better backbone for our public transport services,

"The capacity HS2 is going to deliver is absolutely crucial."

Many smaller businesses have also welcomed the plan. Lee Kemp, who runs Vermillion Films in Birmingham, which makes corporate training films, said HS2 is "really good news for us as a city and as a region".

"With this actually moving forward people can start making plans around it," he told the BBC.

However, the building of HS2 has also had its casualties.

Ron and Anne Ryall have been ordered to leave their home in March, because it is on the planned route and is due to be demolished.

Ron told the BBC: "It's completely wrecked our lives. I'm finding it difficult that someone can just walk into your life and destroy it. My family has lived in this lane for 100 years. I was born here."

'Level up'

A source close to the project has confirmed that Boris Johnson will also announce a series of other transport projects on Tuesday. The projects are aimed at answering critics who fear the high cost of HS2 will mean other infrastructure projects are sidelined.

Some of them will be situated in the north of England and the Midlands as part of the government's manifesto promise to "level up" the UK by investing more in regions outside of London.

Media playback is unsupported on your device Media caption HS2: How much work has already been done?

Approving the plan could be seen as a risky move for Mr Johnson as there has been opposition to the scheme within the Conservative Party.

His chief adviser Dominic Cummings called HS2 a "disaster zone" last summer, while several MPs who represent constituencies that could be affected by the building works are also against the project.

Transport Secretary Grant Shapps has defended the right of Tory MPs to oppose the plan, saying they were entitled to represent the views of their constituents.

Former Conservative cabinet minister David Lidington, said he feared HS2 was "20th Century technology and infrastructure for a 21st Century economy".

Labour peer Lord Berkeley - former deputy chairman of the project's independent review - said HS2 would not help commuters in the north, but would be good for people travelling between London and Birmingham.

Environmental groups say HS2 will boost carbon emissions and spoil areas of natural beauty.

In 2015, it was estimated that the cost of HS2 would be £56bn, but a review has warned that it could rise to as much as £106bn.

Work to prepare the ground for the first phase of HS2 linking London and Birmingham has been going on for years. But a green light from the government means construction of the railway will begin relatively soon.

It is also expected that work on the line between Birmingham and Crewe will be rolled into the first phase. It had originally been slated to begin at a later stage.

Construction on the second phase of the project, originally planned to link Crewe to Manchester and Birmingham with Leeds, is at a much earlier stage.

The BBC has been told that the government will commission a further review of this part of the line to find ways of making it more affordable.

It also wants to ensure that these lines can be better integrated into the existing railway network along with other planned upgrades such as Northern Powerhouse Rail, which aims to improve east-west links across the north of England.

Media playback is unsupported on your device Media caption Working on HS2: "The opportunities are just immense"

What is the HS2 route?

The initial plan was for a new railway line between London and the West Midlands carrying 400m-long (1,300ft) trains, with as many as 1,100 seats per train.

The line would enable trains to reach speeds of up to 250mph and would run as often as 14 times an hour in each direction.

This would be followed by a V-shaped second phase taking services from Birmingham to Manchester and Leeds.

Image copyright HS2 Ltd Image caption Early work has begun on the first phase of HS2

The Department for Transport (DfT) has said that the project will triple the capacity of trains across the entire route.

According to the latest estimates, the first stage would be completed between 2028 and 2031 - long after the original opening date of 2026.

Meanwhile, the second stage would be ready in 2035-2040.

The announcement on High Speed 2 broadly mirrors some of the main recommendations from the government's still unpublished review into the project.

I've read the full report and it emphasises that any significant changes to the first phase of HS2, linking London to Birmingham, would have caused further delays.

No-one in government wanted that.

The Crewe to Manchester and Birmingham to Leeds phase will undergo another review because it is at such an early stage, and ministers believe savings can be found. They also want the high-speed lines to be better integrated with our existing rail network and future infrastructure projects.

Politics and pledges to voters in the north of England and the Midlands was a big part of this decision. But so too was the amount of work which has already been done on the first phase of the project.

Ultimately, the government felt that turning back wasn't an option.

Its much higher price tag has made HS2 politically toxic, so the government is also announcing a massive cash injection for buses and cycling in England.

I'm told that figures within government who weren't sold on HS2 wanted those other transport pledges at the same time.

Why have the costs and timetable slipped so far off track?

Essentially, because they were over-optimistic in the first place.

The National Audit Office has warned in successive reports over a number of years that HS2 had an "unrealistic timetable".

In its most recent analysis, it said that the risks of the project were never properly assessed when initial costs were estimated.

HS2 began under the Labour government in 2009, but the cost has continued to balloon under subsequent administrations.

What are the proposed advantages of HS2?

Once it is built, journeys will be shorter. London-to-Birmingham travel times will be cut from one hour 21 minutes to 52 minutes, according to the DfT.

And while it is being built, it is expected to create thousands of jobs and provide a stimulus to economic growth.

Darren Caplan, head of the Railway Industry Association. told the BBC that the line would also "increase capacity on the East Coast Main Line, the Midlands Main Line and the West Coast Main Line. There are benefits all round".