Independence for Scotland will put the rest of the UK "at risk", according to the Scottish Conservative leader.
Annabel Goldie MSP will raise her concern at the UK party's conference in Manchester.
She will be joined in a special session featuring senior Tories from Wales, Northern Ireland and Scotland.
Describing Britishness as a "state of mind", she will implore all delegates to "rally to the cause".
Her comments come against the backdrop of a majority victory at Holyrood for First Minister Alex Salmond's SNP. Mr Salmond is committed to a referendum on independence towards the end of this five-year parliamentary session.
In her speech, Ms Goldie will say: "None of us can sit passively by. Believe me, this matters to us all.
"Because if one bit of our United Kingdom house gets demolished, instability sets in, the rest is at risk. And we are all better off together. Scotland in Britain is good for Britain.
"Being part of a union is about sharing of responsibility, risks and rewards.
"Put at its simplest, being Scottish and British - or English or Welsh or Northern Irish and British - is a state of mind. It's the way we feel, it is the way we are. Proud of our distinctive cultures and traditions, proud of our partnership together. It just is."
Ms Goldie, who announced after the Scottish election that she will step down, is expected to tell Mr Salmond to spell out the terms of the referendum. She will say: "If only he would get on with his separation referendum so that Scotland can speak. And vote to stick with England, Wales and Northern Ireland."