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A review is to examine whether the UK government's Wales Office should be beefed-up, BBC Wales has been told.

Prime Minister Theresa May will stress the importance of the union in a speech in Scotland on Thursday.

Whether the Wales Office - which has limited powers - should have a stronger role in setting UK government policy on Wales is expected to be on its agenda.

The Wales Office was created at the start of devolution 20 years ago and is currently led by Welsh Secretary Alun Cairns.

Its powers are much smaller than the Welsh Government in Cardiff, which controls matters such as health and education.

It works with UK government departments to ensure Wales' interests are fully represented.

Lord Dunlop, the former Scotland Office minister, will chair the review, looking at how the UK government's structures - including government departments - can co-operate to ensure devolution works.

It is expected it will examine the parts of the UK government that cover Scotland, Wales and Northern Ireland, which includes the Wales Office, as well as the role of their respective cabinet ministers.

It could mean Whitehall departments working more closely with the Wales Office.

But a source stressed that it was not about the UK government taking action on things that are devolved to the Welsh Government and the assembly.

"If some politicians say that is a threat to them, the challenge is why is that a threat," the source said.

"Alun is way past wanting to please politicians. It is people not politicians that Alun cares about."

Mrs May will say devolution has been a source of strength for the UK, not a sign of weakness, in her speech in Scotland.

Strengthening the union must continue to be a top priority for the next prime minister, she will say.

The leadership contest to replace her as Conservative leader, and prime minister, is due to end on 23 July.