John McDonnell's comments follow Sir Richard Branson's tweets relating to Jeremy Corbyn's claim that he could not find a seat on a Virgin train, forcing him to sit on the floor, in a video about re-nationalisation of the UK's rail services. The tweets contained pictures and video taken from CCTV on the train, apparently showing Corbyn and his team walking past a number of empty – although reserved – seats.

Branson, who owns Virgin Trains, tweeted an image from the CCTV footage with a link to the company's press release about the incident, which later became known as "Traingate". In the tweet, Branson said: "Mr Corbyn & team walked past empty unreserved seats then filmed claim train was 'ram packed.'"

McDonnell, who is a close ally of Corbyn's, defended the Labour Party leader, who maintained he had been unable to find seats on the train until a staff member intervened later, was critical of Branson's involvement in the scandal.

In an article for the Sunday Mirror, he wrote that Branson was a "tax exile who thinks he can try and intervene and undermine our democracy."

He added: "The whole purpose of the honours system is undermined when the rich and the powerful can collect their gongs without giving anything back. It's even worse when tax exiles are given honours."

Referring to Branson's residence on Necker Island, which the multi-billionaire bought aged 28 in 1978, McDonnell wrote: "It should be a simple choice for the mega-rich. Run off to tax exile if you want. But you leave your titles and your honours behind when you go."

McDonnell was not the only member of Corbyn's team to criticise Branson, with the Labour leader's leadership campaign director, Sam Tarry, telling BBC today: "The bigger story is that a tax exile of more than 10 years decides to lay into [Corbyn] on social media in a very public way.

"Richard Branson is literally laughing all the way to the bank at the British taxpayers' expense. In the last year, this chap has made some £53m directly from the taxpayer."

Not all agreed with McDonnell's stance, however. Corbyn's opponent in the Labour leadership election, Owen Smith, told LBC Radio that McDonnell's suggestion, "seems a bit much, to be honest. I think it was merely pointing out the reality that Jeremy didn't need to sit on the floor".

Corbyn's video was posted as an increasingly hostile situation developed between Southern Rail and its customers. A summer of cancelled services, delays and strikes by the company have caused misery for commuters as its operators have failed to find agreement with trade union RMT.

The local economy was also said to have suffered as the summer's usual tourists stayed away due to the disruption.