The Sun's James Manning revealed that officials' attempts to remove the percentage of votes for candidates from the new counter was easily gotten past.
He showed how the London Elects counter's web page could be manipulated to disclose the percentage of votes cast in favour of front-runner candidates Sadiq Khan and Zac Goldsmith.
By opening the page's source code - a string of characters that translates into what a browser displays - users could discover the numbers behind the un-labelled bar chart.
The figures show that, at the time of publication, the British National Party - removed then reinstated from the official list of political parties back in January - trailing some votes behind the Cannabis Party.
Users can navigate to a panel displaying the webpage's source code by pressing 'Control + U' on a Windows computer or 'Option [Alt] + Command + U' on a Mac.
The simple hack also reveals the results so-far of other elections taking place in London today, including the London Assembly city-wide and district ones.
The official results of London's elections are not expected to be announced until late on Friday evening.