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.”
PA