Labour will suggest the Saltire does not belong simply to the SNP as MPs debate independence today.
The use of the flag at Westminster is a sign that it is also "Scotland's Parliament", Margaret Curran, Scotland's Shadow Scottish Secretary will say.
The mural of St Andrew and the Saltire in the Palace of Westminster "standing alongside those of St David, St George and St Patrick remind me what the United Kingdom is. Four countries together, working in partnership as equals," she will argue.
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The SNP said it welcomed the debate, which it said it would use to show Scots would have the best of both worlds, post-independence.
SNP Westminster Group leader Angus Robertson MP said: "The strong relationship between Scotland and the rest of the UK will continue when an independent Scotland stands alongside the other nations of these islands as a friend and equal."
Scottish Secretary Michael Moore will also use a speech to City UK in Edinburgh to warn that companies could desert Scotland if it becomes independent. He will say the consequences of leaving the UK are based on "enormous doubt".