THE honeymoon is well and truly over. The Labour Party which delivered the Scottish Parliament now finds itself lagging behind its Nationalist rivals for the first time since it won last May's first Scottish General Election.

Labour's fall has been mirrored by a boost to the SNP. This has propelled Mr Alex Salmond's party into the position, on our Scottish Parliament projection, of being the single largest party on 49 seats, with Labour trailing on 42.

This is the first time the party has achieved that feat since the Parliament's inauguration, and for only the second time since System Three began asking separate questions for the two ballots.

The turn-around in fortunes is nothing short of remarkable, with Labour recording its lowest yet share of voting intentions for the Scottish Parliament first ballot, at 33%. Overall, it is Labour's worst showing in our monthly System Three series since the two ballot questions were asked for the first time in July 1998. Labour's second ballot share of 29% matches its lowest yet, recorded last August.

The catalogue of woe continues in our seat projections. With a projected 42 seats, it is the party's lowest yet share of MSPs. The previous low was 46 seats, in July 1998.

These two low points are the only times Labour has failed to reach the 50-seat mark in our monthly series. In contrast, the SNP's projection of 49 seats is its best since September 1998.

All of which adds up to the most serious challenge yet to Labour's dominance in Scotland over the past 25 years. This is not a poll for an institution yet to be in existence. This is a poll which is about the very real struggle for political control and perhaps even the future status of the new Scottish Parliament.

Nor is this poll a flash in the pan. The trend has been clear over the past five months: Labour has lost support month-on-month during that period.

Although not as yet clear, the trend has also been for the SNP's support to be increasing. The SNP may not yet be the natural party of government in Scotland but it is now cemented as the natural party of opposition in Scotland. That is a significant step forward for a party which has yearned for signs of a breakthrough in capturing Labour support.

Detailed analysis of our System Three series over the past five months shows the sought-after breakthrough may be closer than ever for the SNP. The breakdown of our Scottish Parliament projection shows SNP would take 29 first-past-the-post constituencies, no fewer than 20 gains from Labour.

It would mean only two out of six seats in Edinburgh would be held by Labour, while the SNP would take three. Aberdeen and Dundee would both be Labour-free zones and the SNP would take Baillieston and Govan in Glasgow.

In the Central Belt, the SNP would win in places like Dumbarton, Cumbernauld and Kilsyth, Kilmarnock and Loudon, Falkirk East, Livingston, Linlithgow, and Renfrewshire West.

The North-east would have no Labour constituencies at all and, on the second ballot, the SNP would take a higher vote share than Labour in every region of Scotland except Glasgow.

The poll is also a reflection on Labour's poor performance in real polls, such as the recent Ayr by-election.

How hopeful can the SNP be of making the breakthrough against Labour? The evidence is pointing more and more to the SNP's ability to take Labour's core vote from right under its nose.

Analysis of the social class composition of party support for the Scottish Parliament adds to the evidence gathered by the Scottish Election Study team, which identified a drift away of Labour's core support since 1997.

If we look at the traditional high Labour supporting social class groups D and E over the past few months, something dramatic has happened. At the beginning of the year, support for Labour in this key group stood at 50% for first ballot voting intentions. In February, the figure dropped to 45%. In March, it was 41%, and now it stands at 34%.

For the SNP, the trend in these groups is in the opposite direction. Some 34% of social classes D and E supported the SNP in January. The figure is now 42%. Significantly, this month sees the SNP take more of this social class group than Labour for the first time since last May's elections.

The news is also worrying in another key class, C2. This group has often been more volatile in recent years and, in the past, Conservative victories in England were based in some measure of support from it.

In Scotland, the C2 group has tended to go with Labour but now the SNP is gaining here too. Since the beginning of the year, support for Labour in the C2 group has fallen from 48% to 37%. For the SNP, support has gone up from 39% to 46%, with SNP overhauling Labour last month and continuing that trend this month.

Labour is under severe pressure. It is coming under fire on Section 28, the building of the Holyrood Parliament, general mid-term blues affecting the UK situation, and backed up with a fall in its vote share for Westminster.

It has also not reaped the expected gain from a Budget give-away for the public services. It is seeing its core support seep away in Scotland: to the SNP on one side and also to a Scottish Socialist Party, which would pick up three seats on our projection this month.

The fight is now on to see whether the Scottish Parliament will ever see a full second term. Labour would not even be able to form a coalition on the basis of its support this month. The SNP could.

If things get any worse for Labour, then the ''settled will of the Scottish people'' is in very great danger of being torn up.

q Malcolm Dickson is a lecturer in politics at Strathclyde University