DailyStar.co.uk, another Express Newspapers title, also reported growth in April as traffic rose by seven per cent to just over 6.8 million.
Express Newspapers only started registering their website traffic with ABC in February but have seen consistent growth over the last three months, the only audited publisher to do so.
Geoff Marsh, acting group director of digital content at Northern and Shell, which owns Express Newspapers, told Journalism.co.uk the company's board had "significantly increased investment across its digital division last year".
"For the first time we now have a responsive design," Marsh said via e-mail, "ensuring that users on both mobile and tablet can enjoy the Express and Star websites with the same, simple clarity as desktop users.
"We have also simplified and relaunched our free 'Lite' apps on both Android and iOS, which have been very popular."
Editorial output at the Express and Star was also a factor, said Marsh, as newsrooms had been producing stories with a more visual focus in "striking pictures and compelling, relevant video".
"As a direct result, more websites are linking to us, more people are sharing our stories and we're performing much better on search engines," he said.
Metro recorded the largest drop in traffic during April, down by 15 per cent to 22.6 million, as did sister site MailOnline, which saw a decrease of 7.7 per cent, but at just under 166 million unique monthly visitors MailOnline remains the most popular website among audited titles by some distance.
Traffic to the Guardian dropped back below the record 102 million unique monthly visitors set in March to just over 95 million, a decrease of 6.9 per cent, halting a steady run of growth that began in November 2013.
Digital traffic to titles under the Mirror Group had been growing steadily since last October, but this month fell by just under 6 per cent to 48.1 million.
The Telegraph recorded a fall in traffic of a little over 11 percent, from 72.2 million to 64.2 million unique monthly visitors, while traffic to the Independent fell for a second consecutive month, by 7.6 per cent, to 34.1 million.
Update: The above comments from Geoff Marsh were added to the story after initial publication
Correction: The graphs have been corrected to show stats for Mirror Group National titles, and references to the London Evening Standard have been removed in order to focus on national newspaper site results, which has also been clarified at other stages of the article.
Additional reporting by Abigail Edge