Image copyright HS2

MPs say the HS2 rail link needs a "realistic timetable" and believe the current schedule is "overly ambitious".

The Public Accounts Committee says "it is not convinced" that the first phase of the £56bn rail line - linking London and Birmingham - will open at the end of 2026 as planned.

It also urged the government to clarify the route and costs for the second phase, linking to Manchester and Leeds.

A government spokesperson said the project was "on time and on budget".

"We are keeping a tough grip on costs, and pressing ahead with plans for Phase Two - with further details due to be announced this autumn," the spokesperson for the Department for Transport added.

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In its report the committee acknowledged there had been "considerable progress" on HS2 since its first report on the project in 2013.

However, committee chair Meg Hillier said both parliament and the public were "still in the dark about crucial details - not least when the railway will open, how much it is expected to cost and precisely where it will go".

Analysis: Richard Westcott, transport correspondent

The biggest question hanging over HS2 is quite basic really.

Where's it going to go?

The route for phase one linking London to Birmingham has been nailed down, but the part that forks off to Manchester and Leeds is still up in the air.

We were meant to get details in 2014. That slipped to 2015. Now it's "by the end of the year".

The latest proposal, announced just a few months ago, cuts through hundreds of homes on a brand new Doncaster housing estate which isn't even finished yet.

Local MP Ed Miliband (remember him?) is furious about it.

Three years ago I interviewed a Cheshire farmer called John Keleher who wasn't sure if the line would cut through the middle of his land.

He got back in touch recently saying, "obviously we know nothing more today than we did when you were with us nearly 30 months ago, as the decision for HS2 Phase 2b has been put off yet again".

The government's under a lot of pressure to remove at least some of that uncertainty from ordinary people's lives.

The committee said the cost estimates for the second phase for the rail link were "volatile", and £7bn higher than the agreed £28.5bn funding.

It noted that £9bn of potential savings had been identified, but said it was unclear whether these could be made without "adversely affecting the expected benefits of the programme".

HS2's recent recommendation of a new route through South Yorkshire, scrapping a proposed station in Meadowhall in favour of a stop in Sheffield city centre, was cited by the committee as an example of the "significant uncertainty that remains about plans for phase 2".

"Lack of clarity over plans for HS2 in South Yorkshire highlights what is at stake for communities and local economies, and why government must explain its intentions and the basis for its decisions in a transparent manner," added Ms Hillier.

HS2, the firm building the route, said in a statement that it was "making good progress in building confidence in our programme plan, and will continue to do so over the coming months".

The report comes just days after HS2 announced that current boss Simon Kirby would leave the firm to take up a senior post at Rolls-Royce.