Guardian Media Group is planning to cut 250 jobs – including 100 in editorial – and to restructure the less profitable parts of the company in a bid to break even within three years.

As part of plans to cut operating losses, which amounted to £58.6m in the year to the end of March, the Guardian and Observer publisher is to give up its ambitions to turn the Midlands Goods Shed, a former train depot, into a large events space and will restructure the less profitable parts of the business.

In total, the UK workforce is expected to be cut by 18%, or 310 jobs, as about 60 positions in both commercial and editorial remain unfilled. The company hopes all cuts will be made by voluntary redundancies.

In a joint email to staff, editor-in-chief Katharine Viner and chief executive David Pemsel said the “volatile media environment” had led to an “urgent need for radical action”.

“Our plan of action has one goal: to secure the journalistic integrity and financial independence of the Guardian in perpetuity,” they wrote, before adding they hoped the cuts would all be voluntary and that compulsory redundancies would only be considered only “if necessary”.

Brian Williams, Guardian father of the chapel for the National Union of Journalists, said: “We are encouraged by the fact the company is seeking voluntary redundancies and is looking to mitigate potential job losses by finding other cost-cutting measures.”

Michelle Stanistreet, NUJ general secretary, said: “We will oppose any compulsory redundancies. This news, together with the loss of jobs as the Independent newspapers fold, presents a very worrying situation for the future of newspapers.”

The announcement comes after a difficult year for the newspaper industry as huge digital firms such as Google and Facebook take the lion’s share of advertising budgets while the growth of mobile proves harder to monetise than print for news organisations. The final edition of the Independent newspaper is to be published next week, and other newspaper groups are making significant job cuts.

In total at GMG, some 100 jobs are earmarked to be cut from the 725-strong editorial workforce and 150 from commercial departments, support functions such as finance and human resources and other parts of the business.

A spokeswoman for the company said there are no plans to close the Observer, the Guardian’s Sunday sister title.

Half of all GMG costs relate to staff with the current global headcount of 1,960 understood to have grown by 479 since the last round of redundancies in 2012. The 210 people employed outside the UK are not expected to be affected by the latest round of cuts.

All proposals were presented to staff representatives on Thursday, with the consultation exercise set to last for eight weeks. Voluntary redundancy terms are set to be agreed with unions in the coming weeks.

In January, the Guardian unveiled a three-year plan that aimed to break even at an operating level by 2018/19 by focusing on new revenue streams and a new membership proposal as well as a 20% overall reduction in the cost base.

Print advertising fell sharply in the year by an estimated 25%, and although the Guardian beat this market average, the decline was not offset by digital revenues. The business spent almost £80m in cash in the year to the end of March although the cash and investment fund, boosted by the sale of various assets including its stake in Trader Media Group in 2014, stood at £740m in January, down from £838.3m last July.

In January, Pemsel said: “Against the backdrop of a volatile market, we are taking immediate action to boost revenues and reduce our cost-base in order to safeguard Guardian journalism in perpetuity.

“This plan will ensure our business is increasingly adaptable and better able to respond quickly to the pace of change in the digital world.”