Southern Rail infrastructure is in "poor and unreliable" condition and all elements of the system have been under strain, a delayed report into the company's performance says.

A long-awaited report by Network Rail director Chris Gibb criticises industrial action taken by Aslef and the Rail, Maritime and Transport union over the past year in protest at driver-only trains which has caused travel chaos.

But the report, completed last year, also makes it clear that the Department for Transport, Southern's owners Govia Thameslink Railway, Network Rail as well as the unions, have been "well aware" of how much strain the system has been under.

"Some elements of the system have been considered largely as an afterthought, such as train maintenance depots and stations such as (London) Victoria," says the report.

"On Southern, all elements of the system have been under strain: unreliable infrastructure, a timetable that is very tight and with overcrowded peak services, some key stations that are overcrowded, depots that are full and for historic reasons are in the wrong place, and people involved in informal and formal industrial action.

"The system cannot possibly work to passengers' satisfaction with these components in this state."

Mr Gibb says the RMT and Aslef were the "primary cause" for the system integrity to fail, by taking strike action, declining to work overtime and "generally not supporting and undermining" the system.

"Before this formal action, there were clearly unusually high levels of short-term sickness. The action is obviously reducing the service to passengers, but also the needs of every other party in the system.

"If any other part of the system has a fault, the strike and overtime ban magnifies this many times. No element of the system is perfect, and it can all improve.

"But I am convinced by what I have seen that if the traincrew were to work in the normal manner that they have in previous years, the output of the system, a safe and reliable rail service for passengers, would be delivered in an acceptable manner, which would be similar to other commuter rail services in the South East.

"Their action is undermining the system, and its value to the country that funds it through fares and taxes. Whatever their motives, which are debatable, I do not support their action."

Mr Gibb also states that the Government was "already determining the strategic direction" of the driver-only trains dispute.

Aslef announced it is to ballot its members for strikes in a new row over pay, threatening a fresh wave of crippling strikes from the end of July.

A Department for Transport spokesman said: "Chris Gibb's report sets out the reasons why the Southern rail system faced so many problems last year. But it finds the main cause of widespread disruption was union action and unusually high levels of sick leave.

"The report makes absolutely clear that passengers would have had reliable services had staff come to work as normal, despite the other challenges faced by the network.

"Chris Gibb makes a number of recommendations for the network that we are working with the industry to deliver, including spending £300 million to improve infrastructure resilience and establishing a new board to tackle problems ahead of huge upgrades the Thameslink programme will bring in 2018.

"Southern Rail's performance has significantly improved, up by 23% since December, and more services are running on time. But there is more to do and we are committed to giving passengers the reliable services they deserve."

RMT general secretary Mick Cash said. "No wonder the Government have sat on the Gibb report for seven months. It's taken them that long to slice and fillet it into a document that they can spin as an attack on the unions and the staff. Nobody who uses Southern Rail day in and day out will be fooled by this fit-up by Chris Grayling and the minority Tory Government.

"This is a classic case of who pays the piper calls the tune. It's a shame Mr Gibb never bothered to talk to the unions and the staff and has allowed himself to be used by the Government as a human shield over the Southern Rail fiasco.

"The fight for safety and access to services on Southern Rail goes on. Chris Grayling can cut and paste Gibb however he likes to try and prop up the basket-case GTR operation, but the passengers forking out thousands of pounds a year for Britain's worst rail service won't buy it. GTR should be stripped of the franchise with the whole lot taken into public ownership."

Charles Horton, chief executive officer of GTR, said: "We are grateful to Chris Gibb for his thorough review of the challenges GTR has to face on the Southern network. It is the most congested railway in the UK with passenger journeys doubling in just 12 years.

"As he rightly identifies, we took over long-standing problems and a very challenging franchise with unreliable infrastructure and a very tight timetable.

"The only way to address the capacity problem is to modernise infrastructure, trains, systems and working practices. We urge our trade unions to play their part by working with us on the modernisation of working practices and have assured them that there will be no job losses. We have in fact increased staff providing customer service.

"Some of the changes he recommends have already been implemented, such as the provision of an additional £300 million of infrastructure investment and allowing increased engineering hours on the Brighton Mainline, whilst others are currently being considered by the Government, Network Rail and ourselves.

"Performance on Southern has improved for six consecutive months since this report was written. Things are moving in the right direction, and we believe they will continue to do so as our investment and modernisation programme bears further fruit."