The train drivers' union suspended a strike ballot at the country's biggest rail franchise after facing a legal challenge on which door its members used at a busy depot, it has been revealed.

Aslef was balloting drivers at Southern Railway and the Gatwick Express, owned by Govia Thameslink Railway, over claims of a breakdown in industrial relations.

The voting was halted last week as the union was facing a legal challenge.

Members had backed strikes by 4-1 before the ballot was suspended, it is believed.

GTR lawyers are said to have studied the ballot with the "finest of toothcombs" and discovered that in Brighton some drivers work for engineering and others for the main line, and use different doors.

It is understood the company lawyers raised the issue of drivers' legal place of work.

Aslef made no comment.

A Southern spokesman said: "We welcome the decision by Aslef to suspend their ballot notice over a 'breakdown in industrial relations' and that it will not take any further action in relation to the ballot notice, or the ballot. We are continuing to work with Aslef to address their concerns that led to the dispute."

Southern faces strikes next week in separate disputes over ticket office closures and the long-running row over the role of conductors.