Prime Minister John Key almost knocked himself out in his hotel room in Phnom Penh, he said.

He had been bending over to get his cellphone charger and knocked his head on a desk in the room after a round of golf yesterday, he said.

Key told reporters who noticed a small cut and lump on his forehead that he "saw stars and I don't mean Sylvester Stallone".

The incident happened after he had had a round of golf with Trade Minister Tim Groser.

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He was staying at the Cambodiana hotel for the East Asia Summit.