The London office worker who struck George Galloway on the temple with a rubber stress ball has been cautioned by police.

The unnamed 32-year-old man flung the toy at the Respect MP who was touring central London on the upper deck of his open top battle bus.

“It was a direct hit,” conceded a spokesman for Mr Galloway. The permatanned politician was knocked off his feet by the rubbery missile, which was thrown from the first floor window of an office block.

Mr Galloway, who has already bounced back from the twin humiliations of expulsion from the Labour Party and the Big Brother house, returned to campaigning almost immediately.

“There is a nasty bruise on the side of George’s head, but he will continue campaigning for the local elections,” said Kevin Ovenden, a Respect adviser.

“It hit George on the left side of his head, on the temple. He was momentarily dazed and because of the impact of the blow he lost his balance and hit the other side of his head on the side of the bus.”

The missile, which aides described as hollow and slightly smaller than a tennis ball, is believed to have been thrown from an office block on Procter Street, Holborn shortly after 2pm.

Police arrested a man, thought to work at a research company. He was cautioned by officers last night.

A Scotland Yard spokeswoman said: “Officers from Camden borough attended the office block and arrested a person in connection with the incident.”

Mr Galloway, 53, MP for Bethnal Green and Bow, received medical treatment but was not taken to hospital.

He was travelling on his “battle bus” with his granddaughter and a team campaigning for the London Assembly elections when he was struck near the corner of Theobalds Road and Red Lion Square in Holborn.

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