The decision by Virgin Trains to release CCTV images of Jeremy Corbyn on one of its services after he complained on video about the journey being “completely ram-packed” was in breach of the company’s own policies, the Guardian can reveal.

The company handed images, which appeared to show the Labour leader walking past empty seats before sitting on the train’s floor, to the media earlier this week in a move that embarrassed Corbyn.

A leaked internal document warns that CCTV images will only be made available to the media in two situations: where it is “necessary to seek assistance from the public in connection with a criminal investigation” or where it might improve the “safety of the railway or prevent railway accidents of incidents”.

The Virgin policy, written by the company’s emergency planning, fire and security manager, Jim Rawcliffe, adds: “Any such decision to release footage for these purposes will be approved by the head of safety and environment and, where appropriate, following consultation with the relevant police authority.”

The revelation raises questions about why Virgin and its chief executive and founder, Sir Richard Branson, released CCTV images leading to a political row with the Labour leader.

The company published pictures of Corbyn walking past seats that appeared available in order to hit back after he was filmed sitting on the floor saying that his experience was a good argument in favour of public ownership.

The party leader’s claim was derided, with accusations that he had lied about how busy the train really was.

However, leaked emails reveal that the managing director of Virgin Trains East Coast told staff that the controversy had highlighted how crowded services can be, and that finding seats could make customers anxious and stressed.

David Horne also admitted having to stand by a customer toilet for a journey of approximately 130 miles, from Newark to London. He said that was during Virgin’s “ hot seat week”, when directors and managers are banned from travelling in first class in order to “take a hard look at our standard class offer”.

On Friday, he wrote: “Putting politics aside, this incident demonstrates just how busy many of our services are, those in the middle of the day as well as at peak times.”

Horne also said the controversy highlighted the “great job” that Virgin guards did in helping to find seats for customers on busy services, after a family were upgraded to first class so that Corbyn and his team were able to sit down later in the journey.

“We know from our ‘ideal customer experience’ research that boarding a train and finding a seat can be one of the most anxious, stressful times for our customers, particularly if they do not travel by train frequently,” he said.

“Endless rows of seats with reservation labels inevitably can increase the tension. So top marks to our guards who, day in day out, help customers to find empty seats on busy services – on this occasion it was Newcastle guard Ian Mitchell who is due the credit.”

It followed an email in May, when Horne wrote: “This week it has been another hot seat week, a week when directors and managers are banned from travelling in first class and we take a hard look at our standard class offer – to help us look at what we need to do to make it not so standard!

“Myself I’ve stood up with other customers next to toilets from Newark to London, I’ve seen fellow passengers get stressed out over seat reservations and I’ve realised that it is impossible to juggle a coffee cup and a laptop on one of our airline seatback tables!”

The revelations come as the row between Branson and Corbyn intensified after the shadow chancellor, John McDonnell, told the Sunday Mirror that he thought Branson should be stripped of his knighthood because he was a “tax exile who thinks he can try to intervene and undermine our democracy”.

But Virgin sought to defuse the row. A spokesperson did not address the documents obtained by the Guardian, but instead repeated a statement thanking Corbyn for highlighting how helpful Virgin staff had been during his video.

“He’s also right to point out the need to introduce more trains on our route – that’s why we’re introducing a brand new fleet of 65 Azuma trains from 2018, which will increase seating capacity out of King’s Cross by 28% at peak times,” they said.

They admitted it could be hard to find a seat, particularly during sporting events or at certain times.

“Unfortunately we can’t do anything about cup finals or fares regulation, which could spread demand much more effectively if it was less of a blunt instrument. We have discussed regulation with the government at various points over the last two decades and we would be delighted to work with ministers if they were interested in reviewing the fares structure for long distance services, with the aim of reducing the overcrowding that can sometimes occur.”

Virgin Trains is facing an investigation by the Information Commissioner’s Office over the controversy, amid concerns that the publication of images may have broken the rules of the Data Protection Act.

• This article was amended on 29th August. It originally stated that the CCTV footage was release in order to “make a political point”.