I have looked for this a couple of times, but found it last night. From Labour’s Rules, “The Labour Party does not tolerate electoral fraud”
C5.I.B.ii – The Labour Party does not tolerate electoral fraud and no member who has been found guilty of an illegal or corrupt election practice as defined by the Representation of the People Act 1983 and subsequent legalisation may be considered for selection for public office at any level without the prior approval of the NEC.
With of course the mandatory caveat, unless the NEC grants an exception.