The warning, on the eve of the Conservative Party conference, follows the appearance of David Trimble, leader of the Ulster Unionist Party, at last week's Labour conference in Blackpool - the first ever by a UUP leader.
Conservative Central Office is alarmed at the Unionists' increasing desire to get a hearing in Labour circles. Mr Trimble has let it be known that he was impressed by the way Tony Blair dealt with two Labour MPs' invitation to Gerry Adams, President of Sinn Fein, to visit Parliament last month.
The Labour leader threatened to remove the whip from the MPs involved before Mr Adams made that move unnecessary by withdrawing from the event. A source said Mr Trimble was impressed with the determination to throw Jeremy Corbyn - one of the two MPs involved - out of the party, and the sense of "almost disappointment" when it did not happen.
Yesterday Mr Trimble, who plans to attend the Conservative conference in Bournemouth this week, said the Government (which has a majority of one) could not rely on his party's support when Parliament resumes.
If Mr Trimble's party does withdraw its support, Mr Major would probably still scrape through with a very small majority on the Queen's Speech.
The Unionists expect the Government to lose their majority at some point between now and the general election. One Unionist source said: "Mr Major is a good short-term manager, but it will be a very difficult situation for him because he will not be negotiating with one group, but with several, which he will presumably try to play off against each other."
In these circumstances, the nine Unionist MPs face a dilemma on tactics. If they could precipitate a general election it might result in a big Labour majority which would leave them marginalised. On the other hand, propping up an unpopular government could be disastrous as well as alienating the Unionists from an incoming Labour administration whose victory may become increasingly inevitable.
In the short term, the Unionists will be studying closely legislation, which the Government expects to bring forward, to allow decommissioning of weapons in Northern Ireland without those involved being prosecuted.
Privately, Labour whips remain cautious about the prospects of bringing down the Government, despite the new indications of Unionist openness towards Labour. They point out that, in order to defeat the Government, they still, at present, need all the opposition parties to turn out to vote against Mr Major, as well as one Conservative MP.Reuse content