Profits at the book publisher Penguin slumped by almost 50% in the first six months, thanks in part to the runaway global success of EL James's Fifty Shades of Grey, which is published by a rival, Vintage Books.

The runaway success of Fifty Shades and The Hunger Games helped cut sales at Penguin by 4% to £441m and its adjusted operating profit down 48% to £22m.

"They have distorted the overall books market – Fifty Shades of Grey has sold more than 30m copies," said Marjorie Scardino, chief executive of Pearson, the parent group of Penguin.

"The US is the only place we are down," she said. "Those big bestsellers really did take a lot of air out of the market."

Penguin US published 132 bestsellers in the first half of 2012, compared with 157 in the same period last year, while the UK operation matched its strong performance in the first six months of 2011 with 49 bestsellers. Popular UK titles came from authors including Gok Wan, Rachel Khoo, Antony Beevor and Jojo Moyes.

Ebook sales soared by a third and now account for nearly one in five of Penguin's total revenue. In 2009 ebooks were just 2% of Penguin's total revenues, growing to 12% last year.

Scardino said that although the rate of ebooks growth was slowing, she did not have a view on when the increase in Penguin's digital book revenues would begin to flatten out.

The publisher expects a better performance in the second half, with new titles from well-known authors including Ken Follett, Jamie Oliver and Jeremy Clarkson. Pippa Middleton, sister of the Duchess of Cambridge, will also have a book out.

Pearson's FT Group, which publishes the Financial Times newspaper, boosted sales by 6% to £216m and adjusted profits rose 5% to £22m.

Pearson said that digital subscriptions rose by 31% year-on-year in the first half, to more than 300,000, as registered users of its online product rose 29% to 4.8m.

There are 2.7 million FT web app users and mobile devices now account for 25% of traffic on FT.com.

Across print and online, the FT said its total paid circulation stood at 599,000, up 2% year on year. Advertising revenues declined, although the rate of the fall was not disclosed.

At the Economist Group, where Pearson owns a 50% stake, print and digital circulation grew 5% to 1.62m as at 31 March.

Overall profits at Pearson, which also owns extensive educational and professional training businesses, dropped 28% to £59m.

Scardino said that Pearson would pass the milestone this year of more than half of revenues coming from digital and services as against traditional publishing.

"We still publish a lot of books, the FT on paper etc, but it is a milestone for the business," she said. "We are not really withdrawing the FT from print, it is just our customers are choosing digital. There is still a lot of print around."

She added that despite the first half performance, the company remained on track to report record sales and profits for the full year.