Almost 700 terror investigations are being carried out by the security services.
The figures were revealed yesterday after a car ploughed into cyclists outside Parliament in an apparent extremist attack.
Britain was on a heightened state of alert last night, with Theresa May warning that the threat was now 'one of the starkest we have faced'.
Security minister Ben Wallace said the increased danger of attacks was 'here to stay'. Their warnings came as anti-terror police tried to question a suspect over the latest feared targeting of Westminster.
The man – named last night as Salih Khater and said to be a British citizen of Sudanese origin – was arrested by armed police after the car hit cyclists and pedestrians before crashing into a security barrier at around 50mph during the morning rush-hour. Police raided a string of addresses in the Midlands as they tried to discover what was behind his actions. He was refusing to cooperate with police.
Officials revealed that there were 676 live investigations being conducted by the security services and counter-terror police at the end of June, up from more than 500 three months earlier.
Some 13 Islamist plots have been foiled since March 2017 – including one in the past month. A further four far-Right plots have been stopped.
Officials also said that between 2010 and 2017 there were 2,029 terrorist arrests in Britain, but 412 of those were in the year to December – the highest on record.
Mrs May yesterday called on the country to come together as she voiced her disgust at the attack outside Parliament. It follows the Westminster Bridge attack in March last year in which Khalid Masood killed five pedestrians before stabbing to death PC Keith Palmer at the gates of Parliament.
The Prime Minister said: 'For the second time in as many years the home of our democracy, which is a potent symbol of our precious values of tolerance and freedom, has witnessed terrible scenes just yards from its door.
'The threat to the United Kingdom from terrorism remains severe. I would urge the public to remain vigilant – but also to come together and carry on as normal, just as they did after the sickening attacks in Manchester and London last year.
'The twisted aim of the extremists is to use violence and terror to divide us. They will never succeed.'
Mrs May voiced relief that no one had been killed in the latest attack as she praised the 'formidable courage' of the armed officers who apprehended the driver within minutes of him driving into the security barrier outside Parliament.
'My thoughts are with the innocent members of the public who were hurt in this appalling incident,' she said.
The Prime Minister, who is on holiday in Switzerland, was informed of the incident at 8am yesterday – less than half an hour after it took place. She was kept abreast of the situation throughout the day.
Home Secretary Sajid Javid last night returned to the UK from his holidays. A meeting of the Cobra emergency committee was held by security officials in Whitehall yesterday afternoon.
Last night Mr Wallace told 5 News: 'We have seen a shift since 2017 in the numbers of terrorist plots and the number of live investigations that are going on – both from the far-Right and Islamist extremists.' Asked if people should feel worried, he said: 'I think worried is the wrong word. People should realise that this is at the moment going to be here to stay, that the shift in threat… is a phenomena of this generation.'
The Prime Minister's official spokesman added: 'We have been clear that the threat from terrorism is one of the starkest we have faced, the nature of the threat is changing and so is our response.'