LONDON, July 27 (Reuters) - Losses at Guardian Media Group, which publishes Britain's Guardian newspaper, steepened sharply in its financial year to April as advertising revenue fell, but it said a plan to break even by 2018/19 remained on track.

The Guardian has helped set Britain's news agenda in recent years, winning awards and drawing millions of readers to its free-to-access website. But like many other newspapers it has struggled to develop a successful business model in the internet age.

GMG on Wednesday reported an underlying EBITDA loss of 62.6 million pounds ($82.09 million) for the twelve months ended April 3, worsening from the previous year's loss of 38.8 million pounds.

Its losses reflected volatility in the advertising market, said its owner, chiming in with warnings from other British newspapers including the Daily Mail over the impact of declining print and circulation revenue.

Industry experts have said uncertainty linked to Britain's vote to leave the European Union could further dent newspaper advertising revenues as companies defer spending.

The left-leaning Guardian launched a turnaround plan in January designed to break even by 2018/19 by cutting running costs by 20 percent, after its strategy of building one of the most popular free-to-view websites in the world failed to make up for a sharp drop in print income.

GMG, which also publishes the Observer newspaper, said the plan was on track. It had taken action by cutting 250 jobs in March, and increasing revenues from membership schemes, branded content, video and data.

"We remain committed to achieving financial and editorial sustainability through our three-year business plan," GMG's Chief Executive David Pemsel said in a statement.

The Guardian -- which broke some of the biggest stories of recent years including the phone-hacking scandal at Rupert Murdoch's since closed News of the World tabloid, and Wikileaks -- said its reporting of Brexit in June attracted record audiences.

That coverage coincided with a plea to readers from Editor in Chief Katharine Viner to help fund the Guardian's journalism by making a monthly payment.

"If everyone chipped in, our future would be more secure," Viner wrote in late June.

Guardian Media Group is owned by a trust charged with protecting its flagship daily newspaper.

($1 = 0.7625 pounds)

(Reporting by Sarah Young; editing by John Stonestreet) ((sarah.young@thomsonreuters.com; +44 20 7542 1109; Reuters Messaging: sarah.young.thomsonreuters@reuters.net))