U.K. Prime Minister David Cameron is calling on English, Welsh and Northern Irish people to campaign against Scottish independence.

Keeping the four nations of the United Kingdom together is in all their interests, according to Cameron. Scots will vote in a referendum Sept. 18 on whether to leave the union.

“Though only 4 million people can vote in this referendum, all 63 million of us are profoundly affected,” Cameron will say, according to extracts published by his office. “This matters to all our futures and everyone in the U.K. can have a voice in this debate.”

The U.K.’s reputation in the world relies on its united “brand,” the prime minister will say as he seeks to intervene in the debate north of the border. The Conservatives’ unpopularity in Scotland has led Cameron to rein in his involvement in the campaign while nationalists use his government as an argument for independence.

Cameron made his last major speech on Scotland in Edinburgh in February 2012. In 2013 he visited the nation three times on official business as well as staying with Queen Elizabeth II at her castle in Balmoral and vacationed on Jura, an island off its the western coast.

“To everyone in England, Wales and Northern Ireland, everyone, like me, who cares about the United Kingdom, I want to say this -- you don’t have a vote, but you do have a voice,” Cameron will say. “From us to the people of Scotland, let the message be this: We want you to stay.”