A recently erected memorial to civil rights activist Martin Luther King in Washington needs to be touched up, due to a mistake in the inscription, US media reported.

US Interior Secretary Ken Salazar ordered that an ambiguous inscription - a heavily abbreviated quote from a speech by King - be reworded, the Washington Post newspaper reported on Saturday.

The wording had prompted criticism when the nine-metre-high granite monument was unveiled by US President Barack Obama in October.

"I was a drum major for justice, peace and righteousness," reads the current quote on the monument.

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However, King's original words were spoken in the conditional tense: "If you want to say that I was a drum major, say that I was a drum major for justice. Say that I was a drum major for peace. I was a drum major for righteousness."

Critics say the abbreviated version made King sound arrogant.

"I do not think it's an accurate portrayal of what Dr King was," Salazar was quoted as saying by the Washington Post.

The architects of the monument are now looking for ways of adapting the inscription, the paper reported, however they had previously made clear that there was no room for the full quote.

- AAP