Scotland's governing party set out its plans on Tuesday for a referendum on independence but the main opposition parties vowed to block any move to end the 300-year-old union with England.

The Scottish National Party's (SNP) campaign could lead to an increase in the Scottish parliament's limited powers in a challenge to the authority of new British Prime Minister Gordon Brown, himself a Scot.

"Today is ... the start of the next and, I hope, the most dynamic phase of Scotland's programme of constitutional reform," SNP leader Alex Salmond told a news conference.

He published draft legislation that, if passed, would lead to a vote on a "full independent state" and that also called for a national debate on other options for change, including more powers for the Edinburgh-based parliament.

The pro-independence SNP became the largest parliamentary force in May elections, ending 50 years of political dominance in Scotland by Britain's ruling Labour Party.

The three other main parties -- Labour, the Conservatives and the Liberal Democrats -- oppose a referendum on independence and opinion polls show average support for ending the union at about 30 percent.

The parties, in an unprecedented joint statement ahead of Salmond's announcement, said an independence vote would damage Scotland although they agreed there was a case for reviewing the limited powers exercised by the Scottish parliament and executive set up in 1999.

"We are willing to enter into debate jointly about the way in which devolution within the UK can best develop in the years to come," said the statement, which Salmond said was a sign the argument on more powers for Scotland was shifting his way.

The parliament has powers over health and education, but London still rules on defence, foreign affairs and the economy.


BROWN FACES CHALLENGE

The SNP move presents Brown with a challenge. While arguing against independence, he must try to satisfy Scots' apparent desire for more autonomy or he risks a political backlash.

Scottish votes have helped Labour win the last three general elections. An SNP that is popular and regarded as competent could diminish support for Labour and hamper its chances of securing a fourth term in an election due by 2010 but expected sooner.

More powers for the Scottish parliament could also increase calls for a change in the constitutional settlement under which Scottish lawmakers in London can vote on health and education policy for England but English lawmakers have no say over those matters in Scotland.

The SNP said there were three main "realistic" choices for Scotland: the present set-up, extended powers for the Scottish parliament or full independence.

Salmond said he would prefer a referendum on full independence but was open to a "multi-option" ballot that offered Scots the chance to vote for more powers.

Salmond wants control over Scotland's oil and gas revenues, immigration and fisheries policy, among other areas.