The door to peace is ajar. So who will dare to walk through?

The most important round of Anglo-Irish discussions since the 1920s treaty negotiations could follow yesterday's invitation to talks from the Government to Sinn Fein, the IRA's political representatives.

The talks, due to open formally on 9 September and to begin in earnest on 15 September, could conceivably lead to the emergence of a whole new political disposition on the island of Ireland.

But their scope and potential now depend crucially on the attitude of David Trimble's Ulster Unionist party, which is at present reflecting on whether to boycott the talks, to go in and tackle Sinn Fein head-on for the first time, or to take part at one remove.

The party does not share the Government's belief that Sinn Fein could commit itself to wholly political means. But it is also acutely aware that vital negotiations are on the cards and fearful that its voice might not be properly heard in them.

The formal invitation to Sinn Fein was extended by Mo Mowlam, the Secretary of State for Northern Ireland, in accordance with stipulations publicly laid down this spring by Tony Blair. The IRA first complied with the Government's demands for a reinstatement of its 1994 ceasefire, and has since then apparently refrained from all paramilitary activity.

Ms Mowlam said: "I am satisfied there has been a cessation of IRA attacks. Moreover, there has been no evidence of active targeting and paramilitary assaults which can be directly attributed to the IRA."

Her invitation was instantly and warmly welcomed by the Irish government, by John Hume, the Social Democratic and Labour Party leader, and by Sinn Fein itself. Ray Burke, the Irish Foreign Minister, said: "No party should allow its case to go by default. Let everyone hear your case and listen to your concerns so that we might collectively address them. Courage, imagination and compromise will be required on all sides."

Mr Hume said: "This will also require an agreement that respects the identity of both sections of our people. There should be no victories for any side."

Martin McGuinness of Sinn Fein, describing yesterday's announcement as historic, said the sensible thing was "to get into the talks, take away the reasons why people resort to armed force and hope that we can bring about a situation where all the guns, British and Irish, can be taken out of Irish politics".

Irish nationalism thus appeared united in the belief that the time is right for far- reaching negotiations with, for the first time ever, all elements represented at the table. Constitutional nationalists, who have always advocated dialogue, have in recent years been joined in this by Sinn Fein.

But the state of opinion within Unionism, on which so much will now depend, is confused. The Rev Ian Paisley, who as leader of Unionism's fundamentalist tendency has always said he would never sit down with Sinn Fein, accused the Government of a sell-out and denounced the IRA ceasefire as bogus.

But loyalist paramilitary representatives are much in favour of talks, though some of their spokesmen are warning that Unionists have been made uneasy by the speed and direction of government policy. Most senior Protestant churchmen and businessmen favour talks.

This leaves Mr Trimble, as head of the largest Unionist party, as pivotal to the fate of the talks process. He had hard words to say yesterday about the republicans, describing the thought of meeting Gerry Adams, the Sinn Fein president, as "repulsive". He added: "If he showed genuine repentance for the evil he has done, then maybe it would be a different matter. The people put up by Sinn Fein as negotiators all have blood on their hands."

But behind the uncompromising rhetoric, Mr Trimble has been careful to keep his options open. Next week, in an unprecedented departure for a Unionist party leader, he is to meet Catholic bishops as part of a wide- ranging "community consultation" exercise.

Many observers regard the consultations as amounting to cloud-cover for an eventual decision to go into the talks. The most recent signs are that the party may balk at face-to- face negotiations involving Sinn Fein, but may decide on proximity talks.

Yesterday, however, one newly elected MP pre-empted the consultation process to urge a boycott of the talks. The West Tyrone MP William Thompson said most Unionists did not want the leadership to negotiate with Sinn Fein, even via proximity talks.

There will clearly be some tough internal debate before the party's decision is finalised and before the Government knows whether it has to prepare one or more tables for the momentous talks.