A terrorist attack in the UK in the coming months is "almost inevitable" following a rise in the amount of Islamic terrorist-related activity, security chiefs have warned.
Ministers have been warned of a "step change" in the threat of terrorism by security officials and following confirmation that three alleged plots have been stopped by police and MI5 in the past four months.
The warning follows fears an attack was going to take place on Remembrance Sunday, with armed police present at the Cenotaph in Whitehall, London, where the Queen laid the first wreath.
Four men, aged 19 to 27, were arrested under the Terrorism Act after raids in High Wycombe, Buckinghamshire, and West London, on suspicion of launching a terrorist attack using knives or other bladed weapons.
The unnamed source added since the killing of Fusilier Lee Rigby in Woolwich last May, extremists saw that they "only need a kitchen knife and a mobile phone to carry out a high-impact attack".
Home Secretary Theresa May announced in August the terror threat level in the UK had risen to "severe", suggesting an attack is "highly likely". The highest level is critical, meaning an attack is expected imminently.
May said the reason for the increase was due to "developments in Syria and Iraq where terrorist groups are planning attacks against the West".
There have been a number of suspected terrorism-related arrests recently. Raffaello Pantucci, a senior research fellow at the Royal United Services Institute, previously said it is "inevitable" that British citizens who have been radicalised in Syria will return to attack the UK.
However, General Sir Nicholas Houghton, head of the armed forces, dismissed fears of a terrorist attack.
"In an absolutely unreserved way I would say we have just got to keep continuing our normal life," he told The Andrew Marr Show.
"The last thing that we would want to do is succumb to any sense that there is a terrorist threat that is at all going to stop the British way of life."