The UK has been on "severe alert" since 2014, meaning an attack is highly likely. It's the second highest of five possible UK threat levels.
The Times newspaper is reporting that the UK is facing increasing pressure from domestic terror suspects.
Their intelligence sources claim they have identified 350 potential terrorists who have returned to the UK after training and fighting in Syria.
Andy Hayman, who was head of specialist operations at Scotland Yard at the time of the London 7/7 attacks, told the Times that the increase in the number of potential suspects to be monitored was "shocking."
"I recall the number of people we had to keep under 24/7 watch in the wake of the 2005 attacks — it was far, far smaller than this, yet the drain on our resources was huge," Hayman said.
The stark news is the latest of grim forecasts from police and intelligence officials in the UK.
Before stepping down from his post earlier this year, head of London's Metropolitan Police Sir Bernard Hogan-Howe said further terror attacks on the UK are a case of "when, not if."
However, the UK has fared better than many European allies, most notable, France.
The French people have suffered under an onslaught of high casualty terror atrocities over the last three years, including attacks in Paris that killed 130 in November 2015, and a July 2016 Nice lorry attack which mowed down 84 men, women, and children.
Many attacks were perpetrated by assailants who were known to the French authorities. The public outcry that ensued, resulted in a French parliamentary investigation.
In the summer of 2016, it's findings made for grim reading for socialist President Francois Hollande.
The commission identified multiple failings by France's intelligence agencies.
France has six intelligence units answering variously to the interior, defense and economy ministries.
The report found that the multi-layered, cumbersome intelligence apparatus was like an army of soldiers wearing lead boots.
This weekend's French presidential vote will be a litmus test for the French mainstream political establishment.
Back in Britain, MI5, Britain's domestic intelligence organization, believes that there are more than 3,000 Islamist extremists on home soil.
Senior officers fear teenagers are at particular risk of being groomed by a glut of online Islamic State propaganda.
The warning comes after a flurry of police activity in the capital in recent weeks including: the April 24 Westminster attack, which left five people, including a police officer, dead; the arrest of a man armed with knives close to Parliament; an anti-terror raid in north-west London; and a man detained at Heathrow Airport on suspicion of plotting a terror attack.