Boris Johnson is expected to give his backing to HS2 on Tuesday.

Here are 11 key questions about the controversial high-speed rail project.

– What was its original expected cost?

The project’s initial budget was £32.7 billion at 2011 prices.

In 2015 it was allocated £56 billion.

– What is the latest estimate?

A widely leaked review by Douglas Oakervee found it could cost up to £106 billion.

– What about the schedule?

The first phase between London and Birmingham was due to open in 2026, but HS2 Ltd chairman Allan Cook said last year it would be “more prudent” to open between 2028 and 2031.

– How much money has already been spent?

HS2 Ltd has spent around £8 billion.

The money has gone towards the purchase of land and property, ground investigation work, technical designs, IT systems, wages and public engagement.

– What has gone wrong?

Whitehall spending watchdog the National Audit Office found that the project is over budget and behind schedule because its complexity and risks were under-estimated by the Department for Transport and HS2 Ltd.

– Why is it so expensive?

There are several reasons why HS2 is costing so much.

The railway will sit on concrete slab track, which is more robust than traditional ballast but comes at a higher cost.

Putting the line below ground into tunnels and cuttings at several locations reduces the impact on surrounding areas but is more expensive.

Buying property on the route is also incredibly costly.

– What are the benefits of HS2?

One of the main benefits of HS2 will be the increase in capacity it will provide to Britain’s railways.

HS2 Ltd says the high-speed trains will carry more than 300,000 passengers a day, but they will also create space for more services on existing routes such as the West Coast Main Line.

– How about speed?

HS2 trains will operate at up to 225mph, reducing journey times.

Examples given by HS2 Ltd include Manchester-London journey times cut from two hours and seven minutes to one hour and seven minutes, and Birmingham-London trips reduced from one hour and 22 minutes to 45 minutes.

– Who is in favour of HS2?

Political leaders in northern England and business groups claim the railway is vital to boost transport links across the region, providing increased capacity on an overcrowded network.

Construction firms warn that scrapping HS2 would cause major damage to the industry.

– Who are its opponents?

Environmental groups claim construction work will devastate many natural habitats.

Communities living on or near the route, particularly in the Chilterns, are angry at the impact the railway will have on their lives.

Other critics say the project is too expensive, and the money should be used to boost existing transport networks.

– What is the planned route?

Phase 1 is planned to run between London and Birmingham.

Current designs involve a second Y-shaped phase launching in two stages: Phase 2a from the West Midlands to Crewe followed by phase 2b from Crewe to Manchester, and Birmingham to Leeds.

Neil Lancefield is PA Transport Correspondent.