The Conservatives lost votes and seats across the area affected by the Southern rail dispute, according to RMT union leader Mick Cash.
Mr Cash pointed to Tory defeats in Brighton Kemptown, Eastbourne and Croydon Central – home to thousands of commuters affected by the long-running dispute over guards on trains.
And Association of British Commuters co-founder Emily Yates said: “Once again, we see the public in the south reject the narrative that Southern Rail and the government have been pushing over a whole year of commuter suffering.”
Passengers have known for months that the Department for Transport (DfT) has been holding back a report about the chaos on Southern, she said.
She added: “With this staggering result we hope that MPs of every political colour will finally stand up and join our call for its immediate release.”
The report by a senior and long-serving rail industry executive Chris Gibb is believed to be critical of officials for their handling of the Southern dispute.
Ms Yates said: “The Gibb report has been said to be highly critical of the DfT’s role in this affair and its ongoing suppression is insult upon injury for commuters in the south.”
Mr Cash, whose members have been dispute with Southern for over a year, said: “These results prove that the toxic Southern Rail franchise was a game changer in key seats along the routes served.
“RMT is demanding that the axing of the guards is reversed and the union will harass Theresa May and the transport ministers in her minority government every step of the way as we step up the fight to put safety and access to services before private profit and greed.”
Southern said: “We’re not cutting back numbers of on-board staff. We have more – not less.
“Southern has the busiest network in the UK with passengers having doubled in the past 12 years into and out of London.
“Meeting this demand requires modernisation of infrastructure and working practices so we are transforming and future-proofing our network in order to achieve this.
“Passengers need more trains, not cancelled trains. That’s why our modernisation programme is essential and for six consecutive months since we completed our on-board modernisation plans, our passengers have benefited from improved performance.”