David Cameron's promise of upgrade cash for the A47 has been branded a 'pre-election con trick' by Labour's new shadow transport secretary.

Michael Dugher, who took over the brief last week, said the Prime Minister had 'constantly failed' to meet deadlines for improvements, and Labour would set up an 'independent infrastructure commission' to identify where infrastructure investment was needed.

Lara Norris, prospective candidate for Great Yarmouth, said that while she welcomed any progress in making the Acle Straight safer, she claimed today's announcement was not progress.

'This is making the same announcement for a third time - people want to see action not words. At the moment this so-called 'announcement' is just another stop on the road to nowhere.'

The Treasury said it would commission feasibility studies for roads improvements as part of a £100 billion splurge on infrastructure in June 2013 - but the A47 was missing from the list.

But just months after the government's spending blueprint for 2015 to 2020 was announced, transport secretary Patrick McLouglin travelled to Norfolk to announce a study into how improvements on the A47 could be made.

Mr Cameron said today that between now and 2020 there will be £15bn of spending on 100 schemes - including the A47 - but this would be dependent on the Conservatives still holding power.

Local MPs have welcomed the Prime Minister's promise that improvements to the A47 would be announced in the Autumn Statement next month.

Brandon Lewis said: 'I have fought for improvements to the road since becoming the candidate for the area and am very encouraged by the Prime Minister's speech today. We will hear more detail about the proposed improvements at the Autumn Statement in three weeks time.'

While North Norfolk MP Norman Lamb said he had written to the Chancellor earlier this year to support calls for investment in the A47.

'Improvements to sections of the road are long overdue, and I hope that we will see substantive progress in the Autumn Statement next month.

Toby Coke, Norfolk County Council's UKIP chairman of the Environment, Development and Transport Committee, said: 'Investment into improving the A47 is vital and this interim statement is welcomed.

'However, the devil will be in the detail and that won't become apparent until the Chancellor's Autumn Statement. We are seeking a real commitment from the government and anything less than £500m will show that the government is more concerned about funding it's commitments overseas than it is funding vital infrastructure projects at home, like the A47.

'The £500m will cover the cost of the agreed priority programme of improvements between Norfolk County Council and the A47 Alliance, and on top of this we will be looking for a firm commitment to complete the dualling of the A47 from the A1 to Lowestoft in the medium term.'

Mr Dugher said: 'This desperate so-called announcement of promised road improvements includes no additional money and people simply won't fall for it. Cameron should be judged on his record, not on a speech, and his record on road investment has been nothing but a chaotic series of u-turns. 'Cutting investment, then promising to restore it after 2015. Cancelling road schemes, like the A14, then reinstating them. And constantly failing to meet deadlines for the completion of improvements.'

He said Labour would create an independent infrastructure commission to 'end the dither and delay on the decisions'.

CLA Eastern Regional Director Nicola Currie, said: 'After many years of intensive lobbying and with large-scale public support, it is heartening to see this Government take the call for improvements seriously. It is essential for the long-term health of the region that this route is strengthened – we need to cut journey times and improve safety.'

She said: 'While investment in infrastructure is welcome and this is a significant announcement, we need to remember these are Conservative Party plans for the term of the next Parliament.

The CLA said there was no guarantee that any other party would either follow the plans through, or instigate them with as much investment.

She added: 'We have no indication of where the investment would be made, how much it's going to be, and when this could happen.

'There are many stretches of the A47 that need attention – in Norfolk the section between King's Lynn and Norwich, and the Acle Straight, spring immediately to mind – so getting further detail is vital in understanding the impact this will have on the route and the region.'