An announcement on the rail project linking London to Birmingham and Manchester and Leeds is expected by the Prime Minister on Tuesday.

While the whole line will be built, the government will seek a review of the second phase covering the North.

It hopes it will identify cost savings as well as integrating these lines into the existing railways.

A source close to the project has confirmed that Boris Johnson will also announce a series of other transport projects on Tuesday.

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.

Supporters of the HS2 project say it will cut overcrowding on the railways and help to rebalance the UK's economy.

But it has faced criticism from both Labour and Conservative MPs for being over budget and behind schedule.

In 2015, it was estimated that the cost 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 like Northern Powerhouse Rail, which aims to improve east-west links across the north of England.

What is the HS2 route?

The initial plan is 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.

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

According to 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.

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.

Who are its detractors and what do they say?

They are a mixture of MPs and eco-activists.

Some Conservative MPs oppose the project because the route will pass through their constituencies and they think the money should be spent on better local transport links instead.

At the same time, environmental groups say HS2 will boost carbon emissions and spoil areas of natural beauty.