The leader of the train drivers' union has called for Southern Railway to be stripped of its franchise for "failing" to deliver services.

A fresh row flared between Aslef and the company, part of Govia Thameslink Railway, with the union accusing Southern of keeping the number of drivers "artificially low" and of issuing "misleading" statements about pay.

Aslef members are to launch a ban on overtime from June 29 in a long-running dispute over driver only trains, threatening more disruption for Southern's 300,000 passengers.

Aslef general secretary Mick Whelan said: "It is indefensible that GTR/Southern and the inept Department for Transport (DfT) continue to peddle misinformation and attack those whose goodwill they rely on to run their services.

"The company has - with no censure from the DfT - kept driver complements artificially low. The company relies on drivers working voluntary overtime to run at least 25% of its services while we, as a union, strive for full employment to help deliver a first class public service.

"There would be no requirement for voluntary overtime if Southern recruited staff properly - if the company recruited the right number of drivers to run the services it has promised passengers to deliver.

"We are disgusted and will be asking for GTR/Southern to be stripped of the franchise as all stakeholders, employees, the travelling public, and taxpayers we talk to have lost all faith in Southern Fail to deliver."

Mick Cash, general secretary of the Rail, Maritime and Transport (RMT) union, said: "Southern is a company that instead of running a railway decided instead to declare war on its staff and passengers alike.

"We have received reports of appalling and dangerous overcrowding in the severe heat at London Victoria and East Croydon last night, with staff copping the full rage of angry passengers while the company bosses skulk in their air-conditioned offices.

"This scandal should be called to a halt immediately with Southern shown the door and Directly Operated Railways brought in to sort the chaos they have unleashed."

A Department for Transport spokesman said: "We are committed to giving passengers the reliable services they deserve, but withdrawing the franchise from the operator won't address the issues and would cause more disruption.

"Southern Rail's performance has significantly improved since December but there is still more to do to give passengers the service they expect. We continue to monitor the operational and contractual performance of all franchises."

A Southern spokesman said: "We are sorry for the delays experienced by passengers yesterday and this morning.

"Yesterday, Network Rail engineering work overran on Thameslink in the morning, after which there were a number of incidents affecting the train service because of the high temperatures, in particular a grass fire next to the track near Coulsdon which closed the Brighton Main Line for two hours, causing major disruption right through the evening peak and into the night.

"Rails also buckled in the heat in two places.

"This morning, a rail defect was found at Purley Oaks and both Thameslink and Southern services on the Brighton Main Line were severely affected while Network Rail attended to the problem.

"Many of our trains now have air conditioning which has made journeys much more comfortable for passengers. We act immediately on any reports of failures."

A Govia Thameslink Railway spokesman said: "The hostility in Aslef 's language is regrettable.

"We have continuously negotiated with Aslef and made a very generous offer of a 23.8% pay rise if no further industrial action goes ahead.

"We have had no direct response from the union. Comments by Aslef that imply DOO (driver-only operation) was not discussed as part of the pay offer are incorrect.

"For the record, no false claims have been made and it is misleading to imply that driver numbers are there purely for efficiencies. Many of our drivers state their desire to earn extra money and Aslef are aware that the number we employ allows them to do this while at the same time allowing us to run the railway more efficiently.

"This arrangement has worked well in the past. Less than 10% of services are usually covered by drivers working overtime.

"We hope Aslef will put this very generous offer to a referendum and call off their overtime ban so that passengers will continue to benefit from the service improvements that have resulted in the past six months."