Snooping requests made by British authorities to Twitter have more than doubled - but the site rejects about half of them, new figures reveal.
Police and other government agencies asked for information about users on 299 occasions between January and July.
This was up from 116 in the previous six months and more than the total for the whole of the previous two years.
The requests related to a total of 1,041 accounts - almost three times more than the 371 which were specified in requests between July and December last year.
The data published by Twitter showed that it provided some or all of the information requested in 51% of cases.
This was the highest proportion since the site first published its transparency report in 2012.
Twitter said government requests for account information are "typically in connection with criminal investigations".
Requests may be turned down for "a variety of reasons", the site said, adding: "We do not comply with requests that fail to identify a Tweet or Twitter account.
"We may seek to narrow requests that are overly broad. In other cases, users may have challenged the requests after we've notified them."
The surge in requests could be partly due to an increasing trend for terrorist groups such as Islamic State to carry out recruitment and attack planning on social media.
US-based internet giants have come under intense scrutiny over their willingness to work with security services.
Last year the head of listening post GCHQ Robert Hannigan accused companies of being "in denial" of the role their networks play in terrorism and demanded they open themselves up more to intelligence agencies.
Twitter sparked fresh controversy earlier this year when it emerged it notifies affected users of requests for their account information "unless we're prohibited".
The latest transparency report said the UK "remains a top requester", accounting for 7% of the total 4,363 requests around the world.
Overall, the number of requests for account information jumped by 52% compared to the last six months of last year. Twitter said this was the largest increase in requests it has observed.
The United States was the most active, making more than half of all the requests.
Emma Carr, director of campaign group Big Brother Watch, said: "Thanks to the transparency reports of internet companies, we know police are already accessing data with far greater frequency than many other countries.
"If the public are to have any confidence that surveillance powers are being used proportionately, then we should not have to rely on private companies to publish this data.
"The Government should proactively be publishing their own transparency reports, highlighting exactly how many requests are being made, how often they are refused and why."