In an apparent shift in tactics, the so-called Islamic State (IS) group has reportedly urged British jihadists to carry out "lone wolf" attacks in the UK instead of traveling to fight in Syria.

An undercover investigation has claimed that IS already has a number of bombers in the UK and that one potential plot involves this Saturday's VJ commemorations in a targeted attack against Britain's royal family.

Fictional jihadists created by Sky News and a freelance journalist were reportedly sent terror guidebooks by senior jihadists in Syria, including advice on bomb-making handbooks and information on upcoming domestic terror plots in the UK.

By posing as a male and female wannabe jihadists on Twitter and in chatrooms, the investigators said they "gained a disturbing new insight into the extremists' tactics."

After infiltrating the group's "cyber caliphate," the reporters are said to have made contact with "two major players" responsible for training some of those believed to be in Britain.

The first was Junaid Hussain, a 21-year-old hacker-turned-jihadist from Birmingham, who runs IS' information and recruitment arm from Syria.

His wife, Sally Jones, who is a British convert to Islam and deals with recruiting female IS supporters, reportedly asked the female fictional jihadist what she wanted to do in the UK: to cut a head off or blow up a bomb. Jones then gave the reporters instructions on how to construct a bomb.

Stuart Ramsay, Sky's Chief Correspondent also travelled to the Turkish city of Urfa, close to the Syrian border, where he met with a former IS internal security officer.

The man claimed that "four or five" British men have spent six weeks on a series of terror courses and have now returned to the UK on a "mission." He said the attacks would involve the "Kalashnikov, the pistol, the bomb, grenades, and other things."

Sky said it has informed the anti-terror branch of London's Metropolitan Police about the information its report has gathered.

It follows a Mail on Sunday report claiming that an urgent review of security measures are underway for the VJ ceremonies happening this weekend to mark the 70th anniversary of the end of the war.

Investigators believe the plot against the Queen and UK Prime Minister David Cameron involves the detonation of a pressure cooker bomb, similar those used in the Boston Marathon bombing.

In response to the Mail's report, the Metropolitan Police issued a statement reassuring people that they will be safe to attend Saturday's event.

"While the UK threat level from international terrorism remains severe, we would like to reassure the public that we constantly review security plans for public events, taking into account specific intelligence and the wider threat," it read.

"Our priority is the safety and security for all those attending or involved. The public are encouraged to continue with their plans to attend or take part in events as normal."