AN exasperated Jeremy Corbyn has defended his actions in the row over train overcrowding as his rival Owen Smith openly accused him of using the incident as a political stunt to make the "legitimate" case for renationalising Britain’s railways.

With the Labour leadership contest taking on a more acrimonious tone ahead of tomorrow’s Scottish hustings in Glasgow, the MP for Pontypridd was forced to apologise after using the word “lunatic” at a campaign event.

The remark was taken by some as a direct reference to Mr Corbyn. The Labour leader said he deplored such language and his office accused Mr Smith of “degrading” the contest by “descending into personal abuse”. It called on him to apologise and withdraw the remark.

Read more: Virgin CCTV 'shows Jeremy Corbyn walking past empty seats' in packed train claim

The Pontypridd MP offered an apology to anyone who had been offended by the term but insisted it was not a reference to Mr Corbyn.

He explained: "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."

A speech Mr Corbyn gave in London on his proposal to renationalise the NHS was overshadowed by the train overcrowding row, dubbed traingate.

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, the party leader said there were not enough seats for all of his team.

He then again insisted the post-speech press conference was about the NHS. But as further questions emerged on traingate, Mr Corbyn appeared irritated and 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.

Read more: Virgin CCTV 'shows Jeremy Corbyn walking past empty seats' in packed train claim

"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 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 had 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."

Earlier, Mr Smith was asked if he thought Mr Corbyn had been lying about the train incident.

Read more: Virgin CCTV 'shows Jeremy Corbyn walking past empty seats' in packed train claim

“The evidence is there on the CCTV footage,” declared the Welsh MP. “Obviously, Jeremy did have a seat and he went to sit on the floor in order to make a point about overcrowding. It’s a legitimate point to make…”

Asked if he believed the Islington MP’s version of events or that of Sir Richard, he went on: “I’m not quite sure what the Corbyn version of events is because it changed a couple of times yesterday but what is clear from the footage that I have seen is that he had a seat on the train and there were seats on the train and he chose to sit on the floor for the purposes of the video.

“But he was trying to make a legitimate point and trains are overcrowded and we should nationalise them in order to try and make sure that they serve the British people rather better,” he added.

The row over the train incident continued as both leadership contenders made major policy announcements.

Mr Smith 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.

Mr Corbyn set out his plan for the NHS south of the border with a commitment to stop all private sector involvement in health care, ending private finance initiative contracts and guaranteeing bursaries for nurses.

In his speech, the Labour leader insisted free healthcare was a right not a luxury.

He said the service needed to be fully publicly funded to prevent life expectancy being determined by factors such as social class, race, and occupation.

"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,” he declared.

"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," Mr Corbyn added.

In response to his plan to renationalise the NHS, a Conservative spokesman said: "Whoever wins their leadership contest, Labour are too incompetent and divided to build the strong economy a strong NHS needs.

"We have put doctors and nurses in charge of deciding who can best provide the care patients need and the use of outside providers has grown more slowly than under the last Labour government."