Jon WelchOne of Britain's leading political research companies has hit back at Green Party criticism of an opinion poll it conducted in the Norwich South constituency.Jon Welch

One of Britain's leading political research companies has hit back at Green Party criticism of an opinion poll it conducted in the Norwich South constituency.

The poll, commissioned by the Union of UEA Students and conducted by Ipsos MORI, put Labour's Charles Clarke in the lead with 39 per cent, followed by the Conservatives on 20pc and the Liberal Democrats and Greens both on 19pc.

The Green Party cast doubt on its validity, claiming the sample size of 801 was smaller than the recommended minimum of 1,000 for such a poll.

In a press release, it said the result of the poll was 'weakened' since only 362 people agreed to say who they intended to vote for. 'The undecideds were not taken into account in the published results, even though 'undecided' was the most common response,' it added.

The party also claimed results published by the union were weighted for demographic reasons, which included false assumptions about which demographic groups were most likely to vote for which parties.

'The only fair interpretation of this information is that, with so many people undecided, the election is still wide open - but it does show Labour in first place and the Green Party in second,' the press release added.

But Roger Mortimore, head of political research at Ipsos MORI, said it was untrue that the recommended sample size for such polls was less than 1,000, adding that most used a smaller sample.

He said all opinion polls were weighted since an unweighted sample could not be expected to be representative, and that it was standard practice in British opinion polls to exclude voters who said they were undecided from published percentages.

Mr Mortimore said it was also untrue that 'undecided' was the most common response.

'This was a normal opinion poll conducted in the normal way. They are perfectly entitled to doubt whether it gives a fair prediction, given it was done at the start of March, but the criticisms they make of it are misconceived,' he added.

Meanwhile the Clegg Factor seems to be having an effect in the constituency, where odds on a Liberal Democrat victory are shortening.

Their candidate Simon Wright is now the favourite with some bookmakers to take the seat from Labour's Charles Clarke.

Last night bookies Skybet, Sporting Bet and BWin and the betting exchange Betfair had Mr Wright ahead of

Mr Clarke to win, with odds as short as 11/10.

'The changing bookies' odds reflect the feedback that I've been getting on doorsteps from people across Norwich. This is going to be an incredibly close election between myself and the Labour candidate, and every vote will count,' said Mr Wright.

Most bookmakers still put former Cabinet minister Mr Clarke in the driving seat, however, offering odds at around 6/4 on a Labour victory.

The Conservatives' Antony Little is third favourite at a best-priced 5/1, while the Greens' Adrian Ramsay is available at a best-priced 8/1. Any of the other three candidates - Steve Emmens of Ukip, Gabriel Polley of the Workers' Revolutionary Party and Len Heather of the BNP - can be backed at 11/2.