David Cameron has accused Labour of "an immense slap in the face to the West Midlands" after Ed Balls cast doubt on plans to complete the planned high-speed rail line.

Speaking to the Post , Shadow Chancellor Mr Balls said he wanted to build a new line running east to west across the country before completing phase two of the planned £50 billion rail line, which would run from Birmingham to Manchester and to Leeds.

The first phase of the line, from London to Birmingham and due to begin construction in 2017, would go ahead as planned, although Labour will want reassurances that it is being built in the most cost-effective way possible.

But Mr Cameron said it was essential the second phase of the line was built too.

He said: "I would say to anyone who says there isn't a case for HS2, try getting on the train from Birmingham to London or London to Birmingham and see how many people are standing, see how much extra capacity we need.

"So the case is very strong and has been proven over and over again.

"But the very worrying news today is that Labour are calling into question apparently the links between Birmingham and Manchester and Birmingham and Leeds, and this would be an immense slap in the face from Labour to the West Midlands, the East Midlands and the North of the country and I cannot believe they are contemplating letting down British people in vital regions of our country in this way."

And his comments were echoed by London mayor Boris Johnson, who was in the West Midlands to support Conservative candidates in the forthcoming general election.

Mr Johnson told the Post: "I think it is totally deranged. I think it is Labour turning its back on the West Midlands, the Midlands, the East Midlands and the north of the country. Why would you do that?"

Conservative George Osborne announced he would put HS2 at the heart of the party's election campaign on Thursday, April 2.

He will claim that any threat to cancel £21 billion of investment in high speed rail in the north and midlands would cost jobs and undermine the rebalancing of the economy.

Mr Osborne is expected to say: "I am challenging the Shadow Chancellor to reverse his position immediately.

"After decades of the gap between the north and the south growing, it's now starting to close. If that's going to continue we must invest in projects like HS2."

Mr Balls said he would ask Sir David Higgins, the executive chairman of HS2, to examine whether the proposed second phase of the line offered the "best connectivity and best jobs and investment opportunities for Liverpool and Hull and Newcastle as well".

And in the meantime, he would bring forward proposals which are currently at the very earliest stages for a so-called HS3, which would run from the east to the west of the country.

Speaking at Westminster, the Shadow Chancellor said: "I will want to be discussing how we can improve east-west links. I think that is something we get on with quickly while we ask big questions about the second phase of HS2."

He added: "The idea that we wait to do east-west until after we have done the second phase of north-south is topsy turvy. It has no economic or business logic at all."

While Mr Balls' comments have sent pulses racing around HS2 enthusiasts and opponents, Birmingham City Council leader Sir Albert Bore said high-speed rail remained on track.

But Coun Bore refused to openly join the condemnation of his Labour colleague but rather focused on Mr Balls' commitment to a high speed rail network in general.

Coun Bore, who is also transport spokesman for the Core Cities group of councils, said: "The Core Cities of Birmingham, Bristol, Cardiff, Glasgow, Newcastle, Nottingham, Leeds, Liverpool, Manchester and Sheffield are united in pushing for a radical and comprehensive modernisation of the UK's rail infrastructure, based on high speed rail.

"The HS2 Y network, from London to Birmingham and on to Nottingham and the northern cities is the next stage in that modernisation.

"We are pleased that all three main political parties are now in support of that vital investment and that the Labour Party has repeated that commitment this week.

"But it is not the end of our ambitions. We want to see a complete high speed network, including the proposed east-west line across the north and further links across the Midlands and to Bristol, Cardiff, Glasgow and Edinburgh."