Red has long been a colour associated with revolution, radicalism, and socialism. A red flag has long been used as a socialist emblem. To some, the term red flag refers specifically to the national flag of the old Union of Soviet Socialist Republics. Others take the term to refer to any red flag. Until the 1980s, a red flag was used as the logo of the UK Labour Party, but it is more commonly associated with the revolutionary left than with social democratic parties. The Labour Party also traditionally sings the song The Red Flag as party conferences; it was sung at the October 2003 Labour Party Conference for the first time in several years. The song, which was written by the Irish socialist James Connell in the late 19th century, begins as follows:

The people's flag is deepest red,
It shrouded oft our martyr'd dead
And ere their limbs grew stiff and cold,
Their hearts' blood dyed its ev'ry fold.

So raise the scarlet standard high,
Beneath its folds we'll live and die,
Though cowards flinch and traitors sneer,
We'll keep the red flag flying here.

There are some alternate versions (for example, "The workers' flag" is sometimes sung in place of "the people's flag", or "within its shade" instead of "beneath its folds").