More than a quarter of a million voters in Birmingham have their names on an “open” version of the electoral register - meaning their details can be bought by any company.
Any adult who is entitled to vote must add their name to the electoral register if they are requested to do so.
But while all names and addresses appear on the “full” register - accessible only to election officials, the police, government departments and credit-check firms - more than one-in-three are also on an “open” register.
This is an extract of the full register which contains names and address, and which can be freely bought.
Direct marketing firms, lobbying companies, religious groups and estate agents are among those known to have purchased the open register.
Data published by the Office for National Statistics today showed there were a total of 687,172 people registered to vote in Birmingham at the end of 2018.
Some 430,929 of those had opted out of being on the open register - leaving 256,243 whose names and addresses are available to buy.
Data on how many people have bought access to the open register is not routinely published.
But a Freedom of Information request in 2013 found councils across Britain had sold their open registers nearly 3,000 times between 2007 and 2012, generating around £265,000.
Councils say the registers are expensive to maintain, and they do not make a profit from them.
The proportion of voters in Bath and North East Somerset whose names are on the open register dropped from 38.5 per cent in 2017 to 37.3 per cent in 2018.
People are given the choice of opting out when they register to vote - but can also contact their council at any time to remove their details.