The Islamic State had reportedly planned to carry out a terror attack on the UK Armed Forces Day parade in London on Saturday, but the plan was foiled after Isis unwittingly revealed the plan to a British newspaper.

The Isis plan was reportedly to target the forces and civilians during the parade in Merton in South West London.

The planned attack was aimed to "hit the soldiers in their own land", The Sun reported, adding that it had helped foil the plan after it was revealed to its undercover reporter.

Isis plotter Junaid Hussain reportedly told the newspaper's reporter through an online message to 'jump into the crowd and detonate the bomb.'

"It will be big. We will hit the kuffar (unbelievers) hard InshAllah. Hit their soldiers in their own land. InshAllah. Soldiers that served in Iraq and Afganistan will be present. Jump in the crowd and detonate the bomb," Hussain, who is said to head the Isis hacking wing CyberCaliphate, said.

The Sun then alerted the security services and the anti-terror police and helped thwart the terror attack.

"He ordered our team to target today's parade in Merton, South West London — selected as closest to the Woolwich barracks where drummer Lee Rigby, 25, was murdered by Islamist extremists in 2013," The Sun reported.

The Raqqa-based Isis plotter, who had reportedly been arrested for hacking former UK prime minister Tony Blair's emails, had interacted with the reporter on 1 June and had asked him to carry out terror attacks in London to get an "easy ticket to jannah (paradise)".

The newspaper decided to play along to elicit more information of the planned attack.

Hussain also instructed the reporter on how to build a 'powerful bomb and detonator', even sending him a bomb-making manual.

The manual reportedly listed grotesque methods of increasing casualties and even advised that the bomb's shrapnel be sprayed with rat poison to cause internal bleeding among victims.

The London police have now called on people to attend the event after taking security measures.

"The police, together with our security partners, remain alert to terrorist threats that may manifest here or where individuals overseas may seek to direct or inspire others to commit attacks in and against the UK. It is always helpful when journalists share with us information, as The Sun did in this case, that could indicate terrorist or criminal activity," a Scotland Yard spokesman reportedly said.

Isis is feared to have planned several deadly terror attacks to coincide with the month for Ramadan after its spokesperson Abu Muhammad al-'Adnani had last month asked supporters to take up jihad 'to draw closer to Allah'.

On Friday, Isis claimed the deadly attacks in Kuwait and Tunisia that killed more than 60 people.