Blog: David Cameron argues Scottish independence would mean border controls with England

At the start of his speech to the Scottish Tory conference, David Cameron reminded his audience he had been doing a bit of travelling this week.

He'd been in Germany and Israel to name but two countries. Now he was in Scotland to get back to domestic politics.

And it was back with a bang to perhaps the most pressing domestic issue of them all, the referendum in September on Scottish independence.

The Prime Minister made the case for Scotland remaining inside the United Kingdom.

That's not a surprise. He is, after all, the leader of the Conservative and Unionist party - to give it its official title.

What has provoked debate with the Scottish National Party is the way he made his case and what he did, and allegedly did not, promise.

• Border controls at Gretna?

In his interview with me, for ITV Border's Lookaround, Mr Cameron claimed that were Scotland to become independent - and had to conform to EU rules on movement between countries - then "that would mean a border between Scotland and England".

The Prime Minister said that one of the reasons he opposed independence was he he did not want that to happen.

In response, the SNP point out that there is already a national border between Northern Ireland, part of the UK, and the Republic of Ireland, an independent state.

And, the SNP say, there is a common travel area which allows people to move between the two countries freely.

It will be the same if - 'when' in their eyes - Scotland is independent.

For those in the south of Scotland, and the north of England, this is a key area of debate.

• Greater responsibility

More broadly, Mr Cameron's speech was notable for its claim that the Tories believe in giving the Scottish parliament "greater responsibility for raising more of the money it spends".

When I asked him about whether this meant giving Holyrood power over income tax he did not answer the specific question.

There is a commission under Tory peer Lord Strathclyde looking into this, the Prime Minister said.

However, a line in his the passage of his speech on further powers for Holyrood has been pounced on by the SNP.

Mr Cameron said this: "Vote yes - total separation. Vote no - that can mean further devolution."

To the SNP the word "can" rather than "will" is significant.

According to SNP sources I spoke to this is an important use of language. It means, they say, that this is not a firm promise.

It make Mr Cameron's promise "weak as water" according to the SNP.

Tory sources dismiss that interpretation. They say that Mr Cameron was sincere in his promise though we will have to wait until May to see the detail.

So, the Prime Minister ended his week in Scotland and, as ever, sparked a heated political debate over the constitution.

Voters in Scotland will have to decide which side of these two arguments - two among many - they fall on.