Labour has edged ahead of the Scottish National Party in the race to control Holyrood, according to an exclusive poll for The Herald.

However, with the race seeming even tighter than before, the survey has also blown it wide open by showing the unusually high proportion of people who are undecided.

Among those saying how they will vote, Labour came in ahead of the SNP by four points on the constituency vote, and two points ahead on the regional choice.

Even with that good news for Labour, the survey showed Jack McConnell trailing behind Alex Salmond by 10 points on who would make the better First Minister. Public opinion on independence remains sceptical, with opposition outweighing support by 56% to 44%.

Today's findings run counter to repeated poll leads for the SNP over recent weeks, which put the Nationalists as much as 10 points ahead. They show the race between the two front runners is getting tighter as voters focus on their options.

The telephone survey of 1000 people by mruk research, one of Britain's biggest independently-owned market research companies, found more than half of those polled throughout Scotland said they do not know how they will vote. Yet there are also strikingly high numbers of people who say they are certain or very likely to cast a ballot.

Among the undecided respondents, more than half said they were certain to vote and two-thirds are either certain or very likely to cast a ballot. With the Labour-SNP gap so tight, that large number of potential voters will probably decide the election.

On voting intentions for regional list MSPs, Labour was ahead by 37% to 35%, meaning both gaps are within the 3% margin of error for polls of 1000 people. The LibDems were on 14% and the Conservatives on 10%.

According to James Mitchell, a professor of politics at Strathclyde University, a calculation of how these might translate into seats would put Labour on 55, up five from their tally of seats at the 2003 election, the SNP would be on 45, up 18, while the LibDems would be up one on 18, the Tories would be down seven on 10 seats, and the Greens would secure only one seat, whereas they won seven in 2003. The poll, however, is national and fails to register local regional differences, so seat projections are less meaningful than voting intentions.

According to Professor Mitchell, an average of five recent surveys by different polling organisations gives the SNP a five-point lead over Labour on the constituency vote, and a three-point lead on the regional vote, with Conservatives on 12% for both votes and the LibDems on 15% and 14%. The Greens average 6% and others, including the socialist parties, are on 7%.

The average projections based on these figures show the closeness of this election campaign, with the SNP on 47 seats to Labour on 44, the LibDems on 19, Tories on 14 and the Greens on five. That would deny a majority to a continued partnership of Labour and LibDem, and an SNP-Lib Dem coalition would have a majority of only three.

Labour is aware that much of its traditional support base is reluctant to vote. The party is concentrating its attack on the SNP in order to stop those people switching. It is particularlyconcerned about the middle-class voter base which began to back Labour over the past 20 years.

While the findings suggest Labour could have closed the gap with its main rival, they also suggest Mr McConnell could be a drag on views of his party. Asked who would make the best First Minister, the Labour leader trails Mr Salmond by 47% to 37%.

However, those totals have stripped out those who do not know or think it should be none of the four main leaders. The "none of the above" option gained more support than Mr McConnell.

Among the decided, 10% chose Tory leader Annabel Goldie as best First Minister, and LibDem leader Nicol Stephen mustered 6%.

The focus on the Labour-SNP battle appears to have squeezed other parties. Setting aside half of those who remain undecided, Labour was ahead by 38% to 34% on voting intentions for the constituency vote. LibDems were on 15% and Conservatives on 10%.

The poll, carried out from March 22 to 27, asked how people would vote in an independence referendum. After discounting 17% who did not know, 46% backed independence and 54% were against, in line with recent polls.

A further question asked about voting intentions for local council elections on May 3. Under a proportional system, voters will rank candidates in order of preference. Those who have decided their first vote gave Labour 40% support, the SNP 34%, LibDems 14% and Tories 10%.

This suggests majorities for Labour in and around Glasgow, but not by much.