Jeremy Corbyn walked past empty seats because he wanted two together so he could talk to his wife, he said in response to the row over train overcrowding.
CCTV images released by Virgin Trains appeared to show the Labour leader walking past vacant, unreserved seats before he was filmed sitting on the floor, complaining about “ram-packed” carriages.
Mr Corbyn was unimpressed as he was asked to clarify what happened on the three-hour 11am Virgin Trains service from London King’s Cross to Newcastle following a speech on his plans to “renationalise” the NHS.
He also condemned the use of the word “lunatic” by leadership rival Owen Smith, who appeared to use it when describing Mr Corbyn - a claim the Pontypridd MP later denied.
Asked by one reporter if he was a “liar” over the train row, and a “lunatic”, Mr Corbyn replied: “I was hoping you were going to ask questions about the National Health Service but sadly you’re not.
“I deplore the use of that language in any context; I don’t use it myself and I don’t use it today.”
Explaining the train incident, Mr Corbyn said there were not enough seats for all of his team.
He again insisted Wednesday’s press conference was about the NHS.
As further questions emerged on the train issue, Mr Corbyn later said: “Can we move on, please? We’re discussing the NHS today. Can we have an NHS question, please?”
However, he then relented and was pressed about why he did not sit in unreserved seats which CCTV footage suggested were empty.
He replied: “I’m glad you’ve watched the CCTV so carefully. It’s a really important issue this, absolutely crucial to the future of the whole nation and the NHS.
“But let’s get to the details of it - yes, I did walk through the train. Yes, I did look for two empty seats together so I could sit down with my wife to talk to her. That wasn’t possible, so I went to the end of the train.
“The train manager, who was a very nice gentleman, came along and we had a chat about the problems of overcrowding and regulations on the trains, and he said he’d see what he could do.
“After he’d already offered me an upgrade to first class, which I’d declined, he then very kindly did find some seats, and after 42 minutes I went back through the train to the seats he’d allocated. We sat down there and we then conducted a lot of preparatory work for our visit to Newcastle.”
Virgin Trains has previously said it was “puzzled” by the suggestion Mr Corbyn was unable to find unreserved seats when he boarded the train because CCTV images appeared to show “they’re right next to him”.
Sir Richard Branson, who co-owns Virgin Trains with Stagecoach, also got involved in the row on Tuesday, posting a link to the CCTV images on his Twitter account.
Asked why the billionaire has decided to make an issue of seating on the services, Mr Corbyn replied: “I’m very pleased Richard Branson has been able to break off from his holiday to take this issue as seriously, with the importance it obviously deserves.
“I hope he’s very well aware of our policy which is that train operating companies should become part of the public realm not the private sector.”
The Information Commissioner’s Office (ICO) confirmed it is probing the release of the CCTV images by Virgin Trains.
An ICO spokeswoman said: “We are aware of the publication of CCTV images of Jeremy Corbyn and are making enquiries.
“All organisations have an obligation to comply with the Data Protection Act and must have legitimate grounds for processing the personal data they hold.
“Where there’s a suggestion that this hasn’t happened, the ICO has the power to investigate and can take enforcement action if necessary.”
Meanwhile, the leadership race took a dramatic turn on Tuesday evening when Mr Smith appeared to describe Mr Corbyn as a “lunatic” at an event in Hammersmith.
He told party members: “What you won’t get from me is some lunatic at the top of the Labour Party.”
Allies of Mr Corbyn called for Mr Smith to retract his remarks and apologise to people suffering from mental illness.
But Mr Smith sought to clarify his comments, telling the BBC Radio 4 Today programme: “I was saying that I wasn’t a lunatic.
“Having been accused earlier in the evening of running around like a lunatic, I was saying I wasn’t a lunatic, but if anybody’s offended by the use of that word then I do apologise and I’ve done that already this morning and I’ll do it again.
“But I wasn’t talking about Jeremy. I was talking about me.”
The dispute over language emerged as both leadership contenders made major policy announcements.
Mr Smith has promised to attempt to block the triggering of formal negotiations to leave the European Union until Prime Minister Theresa May promises a second referendum or general election on the terms of the final Brexit deal.
Meanwhile, Mr Corbyn set out his NHS policies with commitments including ending private finance initiative (PFI) contracts and guaranteeing bursaries for nurses.
In his speech outlining plans to “renationalise” the NHS, Mr Corbyn insisted free healthcare is a right not a luxury.
He said the service needed to be fully publicly funded in order to prevent life expectancy being determined by factors such as social class, race, and occupation.
Mr Corbyn said: “We want to ensure that money goes to patients, not private contractors. The NHS will be given resources to provide the world-leading service that we all deserve.
“You can take a journey on the Jubilee Line in London and for every stop going eastwards you can take at least a year off your life expectancy. That is how dramatic the health inequalities are.
“Illness is neither an indulgence for which people have to pay, nor an offence for which they must be penalised, but a misfortune, the cost of which should be shared by the entire community.”