Image copyright PA Image caption George Osborne made a trip on a high-speed rail line during his visit to China

Chancellor George Osborne has urged Chinese investors to bid for contracts to build HS2, as he opened the bidding process for the high-speed rail line.

Speaking in China, he urged firms to bid for seven contracts worth £11.8bn in total - covering the first phase of HS2, between London and Birmingham.

Mr Osborne also invited bids for £24bn of investment in northern England.

Critics say opening HS2 bidding before Parliament has approved the scheme "smacks of a mercenary approach".

It comes as shadow transport secretary Lilian Greenwood said Labour would nationalise the railway system - including HS2.

Speaking about new Labour leader Jeremy Corbyn - who has previously voted against HS2 legislation - she added: "He supports high-speed rail, I support high-speed rail, and the Labour Party supports the continued development of HS2."

What is HS2?

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

They would initially operate at speeds of up to 225mph (362km/h), potentially rising to 250mph (400km/h) and would travel up to 14 times per hour in each direction.

This would be followed by a V-shaped second phase taking services from Birmingham to Manchester and Leeds. Intermediate stations in the East Midlands and South Yorkshire are also planned.

Read more: High-speed rail's long journey

Can the UK be China's best partner in the West?

'A golden era'

Mr Osborne, who is on a trip to boost trade links between the UK and China, announced the start of the procurement process for bridges, tunnels and earthworks.

The chancellor quoted Treasury analysis suggesting 265,000 jobs in Britain "only exist because of our links with China", adding the bidding process would "propel HS2 forward".

The first phase of the proposed high-speed rail link would be constructed between London and Birmingham, with proposed extensions to Manchester and Leeds.

Simon Kirby, chief executive of HS2, said Mr Osborne's appeal was designed to bring "the best technology into the UK", saying the project would boost British jobs.

Final contracts would not be signed until the bill received Royal Assent, he added, saying it was "best practice" to get the firms who will build the track involved in early planning stages.

Image copyright Grimshaw Architects/PA Image caption The plans would see 11 platforms for HS2 at a redesigned Euston station

But Richard Houghton, from campaign group HS2 Action Alliance, said: "Putting contracts out to tender prior to the legislative process being complete smacks of a mercenary approach to HS2.

"Surely the honest and transparent approach would be to only put out tenders after the hybrid bill has been passed and the funds are available."

Campaigners are fighting the plans on grounds of cost and the damage they say the network will do to the countryside it goes through.

Image copyright PA Image caption An artist's impression of how the HS2 train will look

Mr Osborne also invited Chinese investors to become involved in infrastructure schemes in the north of England, including a proposed Science Central development in Newcastle and the Atlantic Gateway development between Liverpool and Manchester.

Communities Secretary Greg Clark said: "Today's pitch book sets out in black and white to Chinese investors over £24bn worth of exciting investment opportunities, which together will deliver key infrastructure projects, help develop a highly skilled workforce and create thousands of jobs."

Other announcements included:

The potential doubling of enterprise zones in the north of England

Efforts to reinvigorate link partnerships between northern and Chinese cities

A new Lancaster University service to bring together UK and Chinese universities, researchers and businesses

Mr Osborne's China trip comes ahead of a state visit to the UK by Chinese President Xi Jinping next month - the first by a Chinese leader for a decade.