The number of terror suspects being arrested in the UK has reached record levels, with 299 arrests in 2014 - the highest level since 9/11, Home Office figures reveal - but only around a third of those held were charged.
The number of people arrested on terrorism-related offences to March jumped 31% compared to the previous year.
More women were arrested than ever before, with 35 held under the Terrorism Act, while the number of 18 to 20-year-olds suspected of terror offences more than doubled, from 20 to 43. Those arrested over the age of 30 also increased by a third from the previous year.
The spike in female arrests was attributed to a surge in arrests in the six months to March 2015. Eight of those arrested were under 18. The female arrest figures have more than trebled in five years.
Of the 299 arrests, 140 (47%) were of people of Asian ethnic appearance, up 36% on the previous 12 months.
The latest figures are the highest since officials began collecting the data in September 2001, and higher than the previous peak of 284, recorded in 2005, the year of London's July 7 bombings.
Officials say there has been a "marked increase" in the number of those arrested who consider themselves to be of British or British dual nationality.
In 2014/15 they accounted for more than three-quarters of those detained for terrorism-related offences, compared with just over half - 52% - in the year to March 2011.
A large amount of the overall increase was driven by a spike in the last three months of last year, when there were 106 arrests, the Home Office said.
The rate fell from January to March this year, with 67 terror-related arrests - but this was still higher than the same period in the previous two years.
A Home Office statistical bulletin said terrorism-related arrests had "fluctuated" since September 11, 2001 when the World trade Centre was attacked.
The bulletin said there was an "initial spike" in arrests soon after the US attacks and around the time the US-led invasion of Afghanistan began the following month. Arrests also increased in the period "immediately after" the London bombings, "where the number of arrests was at its highest since the data collection was established".
Once it peaked, the Home Office said arrests then declined until late in 2010 when the Arab Springs uprising led to figures trending upwards again.
The bulletin said: "Throughout the years since the initial uprising, the number of terrorism-related arrests in Great Britain has seen a steady rise."
The total number of terror arrests in the UK since 9/11 is just short of 3,000 - about 214 a year.
Senior officers have recently warned of a step change in the terrorism landscape, with women and children increasingly at risk of radicalisation.
There has also been a significant increase in the number of suspects arrested who were aged 30 and over, with a rise of more than a third compared to the previous year.
Of the 299 people arrested in 2014/15, less than half - or 118 - were charged with an offence, 100 of which with terrorism-related offences.
The Home Office said this suggested "that police were more frequently able to find evidence to support the link to terrorism following a terrorism-related arrest".
It is likely that the volume of terrorism arrests has risen further since the period covered by the statistics, it said.
Last month Mark Rowley, the country's leading counter-terrorism officer, disclosed that suspects are now being held at a rate of more than one a day.
Fifty-six of the arrests related to international-related terrorism - defined as "activities linked to or motivated by any terrorist group based outside the UK". This was more than a third (35%) higher than in 2013/14.
The number of suspects arrested over domestic terrorism - where there is no link to Northern Ireland or international groups, more than doubled from 15 to 32.
Security Minister John Hayes said the government was determined to detect and disrupt all terror threats to the UK.
He added: "The figures released today once again highlight the hard work carried out by the police, Security Service and Crown Prosecution Service to keep the public safe – and emphasise the scale of that challenge.
"At a time of very significant threat, it is vital they have the powers they need to protect the British public."
Laws rolled out earlier this year have "bolstered our already considerable armoury of powers to disrupt the ability of people to travel abroad to fight, reduce the risks they pose on their return and combat the underlying ideology that feeds, supports and sanctions terrorism," he said.
Hayes said the forthcoming Investigatory Powers Bill, covering the controversial issue of security services' access to communications data, will ensure they are "equipped with up to date capabilities to protect us from those who would seek to do us harm."