Kenny Campbell, editor of the Metro newspaper, has left after 15 years.
Campbell joined the paper as deputy editor as part of the team that launched the title in 1999.
He took over as editor in 2001 after senior roles at the Daily Mail in Scotland, the Daily Express and regional newspapers.
Around 1.4 million copies of the Metro are given away at train stations across the country every weekday (half of them in London). Including its online and mobile platforms it claims to reach around 3.3 million people a week.
Staff were told of his departure this afternoon by Metro managing director Steve Auckland, but the majority of paper staff were not in the office at the time as there is no Saturday edition.
The paper's website, Metro.co.uk, has grown its audience by covering offbeat, shareable news stories but it will now be merged with MailOnline, the Daily Mail digital news operation which is also owned by Associated Newspapers.
Auckland, who returned to the title in November after a stint running local press consortium Local World, said in an interview that despite a growing audience for Metro.co.uk, the brand's focus would be on print, rather than digital growth.
Linda Grant left her role as Metro's MD in November.