The Government is publishing a Hybrid Bill today to get the first phase of the £50 billion HS2 high-speed rail project up and running.
As ministers and supporters of the controversial scheme set out its benefits to the UK, opponents of the project will gather for a rally outside the Houses of Parliament in Westminster.
Today's Bill will, if all goes to plan, enable the building of the first phase of HS2 which is costing £42.6 billion, plus £7.5 billion for the trains.
The first phase, to be completed by 2026, comprises a high-speed line running from London through Tory heartlands in the picturesque Chilterns to Birmingham.
A second phase, to be completed by 2032/33, envisages a Y-shaped extension of the line, taking it from Birmingham to north west and north east England.
Staunchly supported by some and vehemently attacked by others, the project has been dogged by claim and counter claim as to its eventual benefit to the UK on a rail network that is already struggling to cope with passenger numbers.
Today's rally is being organised by the Stop HS2 group with demonstrators expected to come from as far away as Cheshire and Yorkshire.
Stop HS2 campaign manager Joe Rukin said: "With the widespread criticism of HS2 from independent bodies, it is quite depressing that MPs and Lords speaking for the project recently are so ill-informed and unwilling to listen to the exceptionally sound arguments which make it clear HS2 should not go ahead.
"People from up and down the HS2 route are descending on Parliament, not to say they don't want HS2 to come near their homes, but to say that they have studied the plans and justifications for HS2 and that it should be scrapped completely. It is sad that people in affected communities know more about HS2 than the majority of Parliamentarians know about what HS2 means, and we hope to change that."
A Department for Transport spokesman said: "HS2 is absolutely vital for this country, providing a huge economic boost which will generate a return on investment that will continue paying back for generations to come.
"Over the last 15 years the number of long-distance rail journeys in this country has doubled to 125 million a year and without HS2 the key rail routes connecting London, the Midlands and the north of England will soon be overwhelmed.
"This Bill is a crucial step which will give the Government the powers to start construction, get HS2 built and get on with providing the rail capacity this country needs."
Former Labour cabinet ministers Lord Mandelson and Alistair Darling have expressed reservations about HS2 but Labour seems set to back the Bill in Parliament.
Shadow transport secretary Mary Creagh said today: "Labour supports HS2 because we must address the capacity problems that mean thousands of commuters face cramped, miserable journeys into Birmingham, Manchester, Leeds and London.
"However, three years of Government delays and mismanagement has caused costs to balloon. Incompetent ministers have only just launched the consultation on phase 2 of the route, despite the fact that it was being worked on when Labour were in government.
"Our message to David Cameron is clear. Get a grip on this project, get control of the budget and get it back on track."
CBI chief policy director Katja Hall said: "This Bill is a key milestone in delivering an important piece of national infrastructure investment.
"HS2 will tackle the looming capacity crunch on the West Coast mainline, connect some of our biggest cities and bring significant economic benefits. We would urge politicians on both sides of the House to back this important project."
HS2 Ltd spokesman Ben Ruse said: "HS2 is the country's largest ever project. As such it warrants debate.
"However, we must be clear that HS2 will yield hugely significant benefits while addressing the cripplingly strained transport network. Those that oppose HS2 are risking the very future of the country."
Mick Whelan, general secretary of train drivers' union Aslef, said : " We believe Britain needs not only a new high speed rail line but a high speed rail network.
"We would, ideally, like to see HS2 run the length of the United Kingdom, being built from Scotland and the south at the same time, meeting in the middle, linking HS1 (the Channel Tunnel high-speed link) and going via Heathrow.
"The number of passengers on our railways has doubled since 1995 while rail freight traffic has risen by 65% over the same period. The existing rail network is operating at near full capacity and neither new motorways nor domestic air travel are environmentally sustainable options to meet the mobility requirements of a British population expected to grow by 10 million by 2033."
An anti-HS2 group, the HS2 Action Alliance (HS2 AA), said the business case for HS2 had "eight fatal flaws". The group's director, Hilary Wharf, described the scheme as "a £50 billion vanity project project that generations of taxpayers will be paying for with little or no benefit"
A recent report from consultants KPMG said HS2 would boost the economy by £15 billion a year. KPMG bosses defended their report when they appeared earlier this month before the House of Commons Treasury Committee.
However, transport economists told the committee they disagreed with the £15 billion prediction, with one saying the key calculation behind the figure was "essentially made up".
KPMG chiefs are due to appear before MPs again tomorrow - this time before the House of Commons Transport Committee which will also be taking evidence from Transport Secretary Patrick McLoughlin.