Voting takes place next month in marathon polls to pick 41 police and crime commissioners (PCCs) in England - outside London - and Wales.
More than 35 million people are eligible to cast ballots in the November 15 contests.
The Electoral Commission, the poll watchdog, warned today that those not yet on the voting lists have just four weeks to register before the October 31 deadline.
It is seeking to distribute an information booklet to all households in the areas involved.
The commission's head of campaigns Samantha Mills said: "These are new elections, taking place at an unfamiliar time of year and using a voting system that has not been widely used before.
"Our booklet will come through your letterbox at the end of October. It has all the information you need to cast your vote with confidence. This includes what the PCC elections are for, when they are taking place and how to vote."
The commission said anyone who has not yet registered should contact their council's electoral registration officer or visit http://www.aboutmyvote.co.uk to download a form.
The Home Office says PCCs will give the public a voice at the highest level and the ability to ensure police are accountable.
They will aim to cut crime and deliver an effective and efficient service by holding the chief constable to account, setting up and updating a police and crime plan, setting the force budget and precept, and regularly engaging with the public and communities.
They also have power to appoint and - where necessary - dismiss the chief constable.
But it will not be for PCCs to tell the professionals how to do their job.
Among those already announced as candidates are Labour former deputy prime minister John Prescott in Humberside, the party's ex-solicitor general Vera Baird in Northumbria, and former Welsh first minister Alun Michael in South Wales.
Other Labour former ministers listed are Jane Kennedy (Merseyside), Tony Lloyd (Greater Manchester), Paddy Tipping (Nottinghamshire) and James Plaskitt (Warwickshire).
Also standing is Conservative ex-minister Michael Mates (Hampshire).
Labour former deputy chief constable Ron Hogg (Durham) and Tory ex-assistant chief constable John Dwyer (Cheshire) have thrown their hats into the ring.