Chancellor Gordon Brown has told a gathering in Edinburgh that the UK is at its strongest when it is together.
Gordon Brown addressed Labour activists in Edinburgh
The unity-themed speech came the day after Prime Minister Tony Blair announced he would stand aside as leader within a year.
Mr Brown stressed the strong bonds connecting Scotland with England.
He said: "For the first time ever, over half of Scots have relatives from south of the border. We are all part of the United Kingdom."
He added: "The SNP want Scotland separate from the UK and want to force Scotland to choose between Scotland and Britain.
"The Conservatives want English votes for English laws and want the English to choose between England and Britain.
"For all of my political life, I have stood up for Britain and I stand here today again to speak up for Britain and Britishness and for the values that make us proud of our Britishness."
As Mr Brown arrived at the Edinburgh venue he side-stepped waiting reporters' questions about damning comments made by former Home Secretary Charles Clarke.
Mr Clark accused Mr Brown of "absolutely stupid" behaviour during the furore over the Labour leadership crisis.
In his speech to Labour Party activists, Mr Brown said that 2.5 million Scottish residents are either English or have English relatives.
"As Scotland prepares for the third elections to its Scottish Parliament, I believe the people of Scotland will see the positive case for us stronger together, weaker apart," he said.
The Kirkcaldy and Cowdenbeath MP added: "A Britain founded on both the devolution of power but also on a partnership which brings us stability, co-operation and mutual support is the best way of expressing the aspirations of the Scottish and British people."
Scottish National Party leader Alex Salmond said that Mr Brown's words were part of his attempt to "say or do anything" in order to "win the keys to Number 10".
Mr Salmond said: "The more Gordon Brown goes on about his Britishness the more he highlights his own fatal weakness in the Labour leadership campaign.
'Tough on terror'
"He believes that his own Scottishness is an obstacle to him becoming prime minister. As Charles Clarke said, he is 'nervous' and 'lacks confidence'."
President of the Scottish Liberal Democrats, Malcolm Bruce, said that it was right to highlight the changing definitions of what it is to be British and Scottish.
He said his party was best placed to deliver a positive future for Scotland.
As well as delivering his speech in Edinburgh on Friday, Mr Brown wrote an article in The Sun newspaper, headlined "I'll be tough on terror".
He said he would be visiting New York in the next few days where he would be reaffirming to the American people that Britain "under the courageous leadership of Tony Blair" stands now and then "shoulder to shoulder with them".
Mr Brown went on in the article to speak about the painstaking efforts of the security services and police in the war on terror.
He finished the piece by saying: "In the years ahead, all our aims must be a Britain strong in stability, strong in security, and strong in the world - a Britain truly safe and secure in our hands."