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No Undertaking is a legal manouvre, but it is not overtaking on the left. Some European countries such as Spain and Irleand have Undertaking lanes, on the left (right in Spain) where a slow moving vehicle or one that wants to be overtaken can pull into, undertaking is the process of pulling over to let a faster vehicle past. Somtimes the lane is only half a width but the undertaker is indicating to the faster vehicle that it doesn't want to or cannot go faster and that it's ok the go past.
Nearly always in the UK the word Undertaking is used incorrectly.