Until now, polling staff have had the power to turn voters away if they suspected them of being 'idiots'.
'Lunatics' were barred from voting unless they were experiencing "a lucid interval".
But Mencap president Lord Rix's amendments to the Electoral Administration Bill last week mean people can be refused the vote only if they can't give their names or their reasons for being at a polling station.
Laws using the words 'idiots' and 'lunatics' have existed since the 17th century, but most have been scrapped. "People with learning disabilities have come a step closer to inclusion in society," said Jo Williams, chief executive at Mencap.
Scope and Sense also succeeded in getting the Bill amended to prevent people with physical disabilities and communication impairments from being turned away from polling stations.
The amendment affects only electoral law and not political parties, so the Official Monster Raving Loony Party can continue to exist.