A ladder having several collapsible ladder sections (100), each comprising two hollow ladder bars (102, 103) arranged parallel to each other and interconnected by a rung (104), where each collapsible ladder section is telescopically inserted into a lower ladder section. Each ladder bar of a collapsible ladder section has a locking hole (111) and an extension (112) below the locking hole. Retaining mechanisms (106) are provided in the rungs for locking the collapsible ladder sections relative to one another, and the retaining mechanisms comprise a locking pin (108), which can engage a corresponding locking hole provided in the ladder bar of a ladder section positioned there above. At least part of the collapsible ladder sections have a non-locking slot or groove (101) formed in the extensions of the ladder bars, where the slot reaches from a distance below the ladder bar to the bottom of the ladder bar. A ladder bar slot or groove may be positioned in such a way that a locking pin of a below lying ladder bar can pass freely in the slot of an above lying ladder section that is fully collapsed.