A YES vote in the Northern Ireland referendum today would open the door to a new future for the Province, and a furious election campaign for seats in the Assembly.

It would give the go-ahead for a programme which should see Sinn Fein president Gerry Adams in government and terrorist prisoners released on the streets.

For the first time Sinn Fein would be fighting a campaign to take seats in the new Assembly after a change in its rules, and its electoral support is a near guarantee of two executive posts.

But Tony Blair has made it clear that legislation will debar Sinn Fein from government unless IRA violence is over for good. That includes a start in decommissioning IRA weapons.

Under the agreement, prisoners will be released on licence over two years, providing their paramilitary organisation has given up violence. There will be no ''free passes'', and each case will be decided on merit.

A No vote will mean back to the drawing board, and Mr Blair has admitted he has no ''Plan B''.

Legislation enabling the establishment of the Assembly is due to get the Royal Assent within the next fortnight and after a Yes vote fuller legislation will be pushed through the Commons to provide an Assembly with its rules.

The election is expected on June 25 with a total of 108 members returned by PR, six in each of the Province's existing 18 parliamentary constituencies.

In a departure from the norm, candidates will be asked at the time of their nomination to provide a ''top-up'' list of up to three substitutes to the Chief Electoral Officer.

If for any reason a vacancy arises, a by-election will not be held but the named people contacted by the CEO in order of preference until one declares a willingness to take over the seat. The procedure is designed to preserve what is expected to be a delicate balance in the Assembly. Ministers argue that, if a by-election was called, the smaller parties who had won a seat on the fifth or sixth count under the PR system would have little chance of regaining it.

Once the Assembly is in place, members will elect a chair and deputy chair, and establish committees for each of the main executive functions of the Northern Ireland administration, all based on a cross-community basis. They will elect a First Minister - effectively the ''Prime Minister of Northern Ireland''. During a transitional period of up to eight months, local Ministers will ''shadow'' the Northern Office Minister currently running departments.