In the year to March, 299 people were detained for terror-related offences - an increase of 31% compared to the previous year, Home Office figures revealed.
It is the highest number since officials began collecting data in September 2001 and tops the previous peak of 284, which was recorded in 2005/6 in the wake of the July 7 bombings.
The rise was mainly driven by a flurry of arrests in the weeks after the official terror threat level was raised from substantial to severe in August last year in response to the increasing influence of IS, also known as Isil. In the last three months of last year, 106 terror suspects were detained.
There were also sharp increases in the number of women and young adults being arrested by counter-terrorism police, while the proportion of those detained who consider themselves British has reached record levels.
Security Minister John Hayes said the figures emphasise the scale of the challenge for authorities to keep the public safe.
He added: “At a time of very significant threat, it is vital they have the powers they need to protect the British public.”
The data showed that suspects who consider themselves to be of British or British dual nationality accounted for more than three-quarters (78%) of all those detained for terrorism-related offences, compared with 52% in the year to March 2011.
Warnings that women are increasingly vulnerable to radicalisation, particularly by IS, are also borne out by the figures.
A record 35 of the suspects were female - more than three times higher than five years ago - while eight of those arrested were under 18.
The number of 18 to 20-year-olds arrested more than doubled on the previous year from 20 to 43, while suspected terrorists aged over 30 being held increased by a third.
It is likely that the volume of terrorism arrests has risen further since the period covered by the statistics because police said last month that suspects are being held at a rate of more than one a day.
A Home Office statistical bulletin said terrorism arrests jumped after the 7/7 attacks, then fell until the Arab Spring uprisings began in late 2010. “Throughout the years since the initial uprising, the number of terrorism-related arrests in Great Britain has seen a steady rise,” it said.
Other findings include:
• The total number of terrorism arrests in the UK since the 9/11 attacks 14 years ago is now just short of 3,000.
• Fewer than half of those arrested in 2014/15 - or 100 - were ultimately charged with terror-related offences, although this was the highest proportion on record.
• In the year to March 52 defendants were proceeded against for terrorism-related offences, and 42 were convicted.
• 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 2013/14.
• The proportion of suspects held under terrorism legislation before being charged or released for more than seven days was at the highest level on record, 34%.
Hannah Stuart, an expert on terrorism at the think tank The Henry Jackson Society, said the increased number of arrests may reflect both a heightened threat and earlier intervention by authorities.
She said: “The peaks will often come after a major terrorist attack when there is a bigger focus from police and law enforcement.
“In this case I think it is a response to the increased threat and that is a direct response to Isil. That is not to say the other terrorism threats have gone away.”
Police believe at least 700 British extremists have travelled to Syria to fight or join IS, with around half of those thought to have returned to the UK.