Leaders from Scotland, Wales and Northern Ireland have signed a historic pact calling for greater constitutional change, creating a major new challenge to the future structure of the United Kingdom.

The agreement brings together leaders from the three devolved nations around demands for greater self-government and the right to pursue their own constitutional futures. Scotland and Northern Ireland are pushing for independence or reunification votes, while Wales is seeking greater powers.

The development puts fresh pressure on Prime Minister Andy Burnham, whose government has rejected calls for immediate constitutional referendums. The UK government has maintained that major constitutional changes should depend on clear public support.

The joint action could intensify Britain's wider debate over devolution, independence, the Union and the distribution of political power between Westminster and the UK's nations.