THE SNP would be the largest party in the Scottish Parliament, and may be capable of blocking a joint Labour and Liberal Democrat coalition, according to a new analysis of the latest Herald/System Three poll.

It provoked heated political debate yesterday after showing the SNP had opened up a five-point lead over Labour in voting intentions for the Scottish Parliament a year from now. The new study of the effect of the second, regional votes will raise the temperature further.

Scottish Office sources, meanwhile, last night ridiculed the notion that a proposal to bring forward the opening of the Parliament by six months was a panic measure to draw attention away from Labour's slide to 36% in The Herald poll of voting intentions for next year.

Scottish Secretary Donald Dewar announced at Westminster that he was consulting with political opponents about the most appropriate vesting day for the Parliament. At first, January 2000 had been proposed, with the body sitting for several months as a shadow authority before assuming power.

He made clear yesterday the choice now lay between September or October next year, or July.

''Our initial view is that July is preferable,'' he wrote to Opposition party leaders. ''It would keep to a minimum the time during which a Parliament and executive were in existence but had little power.''

But who will form that executive? The assumption has always been that Labour, either alone, or comfortably in coalition with the Liberal Democrats, would form the new Scottish Government, with Mr Dewar a racing certainty as First Minister.

This is suddenly less inevitable, following the trend in recent Herald polls, culminating in a 41% to 36% lead by the SNP over Labour.

The Herald estimated this would produce a breakdown in terms of seats of 53 for Labour and 52 for the SNP, with the Conservative and Scottish Liberal Democrats taking around 12 each.

However, Malcolm Dickson, Strathclyde University politics lecturer, has factored into the original findings the impact of people casting their second, regional list vote differently from the initial first-past-the-post constituency vote.

The results are dramatic: the benefits tend to go towards those parties unfairly dealt with by the constituency vote, making Labour the main loser.

Mr Dickson estimated that, on the first ballot, with 36% of the share, Labour would get 42 seats; the SNP would gain 23 seats with 41%; the Conservatives none in spite of 11%; and the Lib Dems eight seats with 10%.

He has now overlaid that with the kind of second preference switch in voting identified by the weekend's ICM poll in Scotland on Sunday, to calculate that Labour's 31% share of regional list votes would bring an additional four seats.

The SNP, with 44%, would gain 31 additional members. The Lib Dems with 14% would gain an additional 12 seats, while the Tories would gain nine seats from their 10% of second preferences.

This would give Labour 46 of the 129 seats, and the Lib Dems 20: enough to gain an overall majority in coalition by just three seats.

The Conservatives would have nine seats while, with 54 seats, the SNP would be the largest single party, 11 seats short of an overall

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majority and just two gains short of denying a Lab/Lib-Dem working majority. Evidence that around 12% of voters will exercise their second preference differently comes not just from the weekend's ICM poll, but from some internal party canvassing.

SNP constitutional spokesman George Reid, who contested Ochil a year ago and hopes to do so again a year from now, said his local canvassing effort had now covered 2200 voters, many of these questioned on the telephone by party volunteers in London using their new Alpha 4 database.

They were now clear that around 12% of voters were considering using their second preference differently and the SNP would be a key beneficiary of this.

He barely needs it: according to Malcolm Dickson, Ochil would be the SNP's first gain, followed by Caithness; Inverness, Ross and Skye; Argyll; Govan; Kilmarnock; Renfrew West; Dundee East; Western Isles; Tweeddale, Ettrick and Lauderdale; Aberdeen North; Edinburgh North and Leith; Linlithgow; Livingston; Midlothian; and Dumbarton.

Against the background of this potential onslaught on Labour, it was scarcely surprising there was a sceptical reaction to the Secretary of State's proposal yesterday to bring forward the vesting day of the Scottish Parliament, with opponents painting this as a manoeuvre to draw attention away from the implications of the recent poll.

Tory constitutional affairs spokesman Michael Ancram dismissed the move as a ''panic'' measure. ''The Labour Party is severely shaken and not surprisingly Labour is panicking. In my opinion, they have lost control of the devolution agenda,'' he told a Westminster news conference.

Downing Street sources were dismissive of the poll. The Prime Minister's official spokesman said: ''I'm consistent in never responding to opinion polls, be they up or down. I'm equally consistent in saying that once people focus on elections to bodies that have real power, then they may give very different answers.''

Scottish Tories said they would reject the plan to advance the starting date of the Parliament if it was merely ''a quickfire suggestion inspired by panic'', but promised to give it full consideration.

Scottish Liberal Democrat leader Jim Wallace welcomed the move, however, as did the other Constitutional Convention partners, the STUC. Mr Wallace said it made no sense for MSPs to twiddle their thumbs for six months. ''If we are to maintain momentum, it makes sense that the Parliament gets going as soon as is efficiently possible.''

Campbell Christie of the STUC said the promise to get the Parliament up and running within three weeks of next May's election was a tremendous achievement by Donald Dewar, and he made another pitch for the parties to do all they could to ensure gender equality at Holyrood.