Rupert Murdoch has stepped down as a director of News International, in a move that will fuel speculation the media mogul is preparing to sell off his UK newspapers.

In an email sent on Saturday, staff at The Times, The Sunday Times and The Sun were told that Murdoch remained "fully committed" as chairman despite relinquishing positions on a number of UK boards, including NI Group.

The move is in line with plans to restructure his News Corporation empire, the message read. Murdoch has already outlined plans to split the group into two.

Under the proposals, a largely US-based media empire consisting of Fox TV, the 20th Century Fox film studio and other entertainment assets, will be separated from the publishing businesses, including The Wall Street Journal, alongside Murdoch's UK and Australian newspapers and HarperCollins book publishing.

The media mogul plans to remain the chairman of both businesses although Chase Carey, chief operating officer at News Corp, will be elevated to the number two spot at the entertainment business.

In line with that strategy, Murdoch has also resigned from the boards of a dozen companies with interests in the US, Australia and India, it was revealed. It is thought that he resigned as director at some point over the last week.

Staff were informed of the development in an email – seen by the Guardian – that was sent out on Saturday.

It states that the decision "is part of the preparation of the business for the upcoming restructure into two companies".

In a bid to reassure employees amid swirling rumours of a possible sell-off, it adds: "[Murdoch] remains fully committed to our business as chairman of what will become the largest newspaper and digital group in the world and we look forward to seeing him in London over the Olympic Games."

But some staff are said to be concerned that the move is a precursor to putting the UK titles up for sale following a rocky year in which NI has been badly damaged by revelations of widespread phone hacking at its now defunct newspaper News of The World.

Earlier this year a parliamentary committee said Murdoch was "not a fit person" to run an international business.

Media analyst Claire Enders said Murdoch's resignations were part of the "controlled fade of Rupert and James from the UK" that began last year in order to take a "psychological step back" from embarrassments such as the recent phone hacking scandal.

"It's the big farewell, it's time to move on. They are leaving the country and they won't be back. It is quite a historic moment," she said. "This follows on from comments Murdoch made a few weeks ago when he remarked on the unfriendliness of the UK, but it's something that is part of the long-term strategy.

The string of board resignations comes as the 81-year-old faces renewed pressure to resign as chairman of News Corp at the group's annual general meeting later this year.

Last week it was revealed that 18 major shareholders including Connecticut's state pension fund and the UK's Legal & General have now signed a letter calling for Murdoch to step down from the company he has spent 60 years creating.