Parts of the controversial HS2 train line have been rated as "unachievable" by the government's infrastructure watchdog.

As part of an annual report released by the Infrastructure and Projects Authority (IPA), the first two phases on the rail link – from London to Birmingham, and Birmingham to Crewe – were given a 'red' rating, meaning the whole plan may be need to be "reassessed".

This follows reports from earlier this year that the plans for the railway heading through the North West parts of Leicestershire, near to Ashby, could be scrapped.

HS2 is a planned zero carbon high-speed railway line, first proposed in December 2010. It was first expected to cost £33 billion and be open by 2026, but unexpected delays and rising costs meant it is now expected to cost at least £71 billion and open in 2033.

Now, a watchdog has deemed the first two phases of the rail scheme to not be viable, after analysing its construction plans and giving it a 'red' rating. The IPA is a government organisation which reports to the Cabinet Office and the Treasury and said there are "major issues with the project's definition, schedule, budget, quality and/or benefits delivery".

The large rail scheme is being developed in a number of phases, one of which is set to run through Leicestershire. Known as phase 2b, the Eastern Leg will connect the West Midlands to the East Midlands, meaning around 30 km of new railway will pass through the north of the county near to Measham, Ashby and Castle Donington, but stations will not be built there.

Whilst this section of the route has yet to be reviewed by the IPA, the watchdog did rate the further electrification of the Midland Mainline from Wigston to Sheffield and Nottingham as 'amber'. This means the IPA believes the project to be "feasible" but added "significant issues already exist" with the scheme that intends to facilitate HS2 trains to serve the East Midlands.

But the watchdog is hopeful these issues "appear resolvable at this stage" and if addressed promptly, should "not present a cost or schedule overrun."

A Department for Transport spokesman said on the IPA ratings: "Spades are already in the ground on HS2, with 350 construction sites, over £20 billion invested to date and supporting over 28,500 jobs. We remain committed to delivering HS2 in the most cost-effective way for taxpayers. HS2 will bring transformational benefits for generations to come, improving connections and helping grow the economy."