A large-scale uprising from people living under the totalitarian regime of Islamic State (Isis) is the most likely trigger that will lead to the undoing of the self-declared caliphate, according to an authoritative report into the jihadi group by the former head of counter-terrorism at MI6.

A detailed appraisal of the organisation, obtained by the Observer, says that, although Isis has performed strategically well so far, its biggest challenge will be controlling dissent and coping with the difficulties of administration in the vast territory it governs.

The report by Richard Barrett, who headed the UN’s al-Qaida and Taliban monitoring team and helped establish the UN’s working group on terrorism, is one of the most comprehensive portraits yet of the group and examines its genesis, revenue streams and cohort of foreign fighters along with its ambitions.

The social media platforms that Isis has exploited successfully to disseminate propaganda will also play a key role in its demise by rapidly spreading discord among the six million people under its rule, the report states.

It adds: “The thirst for change that Islamic State has managed to exploit will not be slaked by its totalitarian approach towards its subjects. In today’s world, no state, however remote, can hope to control its population by limiting its access to information or suppressing its ability to think. It will be no more able to harness the social, economic, and political forces around it than were the states that, through their failure, allowed the space for Islamic State to grow.”

Barrett said that international agencies had recently noted a “slowdown” in the volume of foreign fighters joining Isis, partly because some that had returned home had talked negatively about their experiences. “The fact that many people have gone home and are starting to talk about how bad things are means there’s a counter-narrative going on which has helped slow numbers,” said Barrett, whose report for security analysts the Soufan Group will be used by governments as an intelligence briefing about Isis.

A report by the UN security council, revealed on Friday that 15,000 foreign jihadis have travelled to Syria and Iraq from more than 80 countries to fight alongside Isis and other groups.

Barrett’s report also quotes the 15,000 figure, adding “over half come from Tunisia, Saudi Arabia, Morocco, Jordan, and Turkey”. However, it adds that, if Isis begins to lose its foreign cohort, it will be weakened to the point it might be overrun. “If these [foreign] fighters desert it, Islamic State will probably be unable to maintain momentum and so be an easier target for its enemies.”

Around 500 Britons are thought to have travelled to Syria and Iraq, although they are not listed among its ranks of suicide bombers in the report which during 2014 has included “Danes, Egyptians, French, Iranians, Jordanians, Libyans, Moroccans, Pakistanis, Russians (Chechens), Saudi Arabians, Syrians, Tajiks, Tunisians, Turks and Uzbeks”.

The UN report on Friday identified the use of social media by Isis as “unhindered by organisational structures”. It is this approach that Barrett believes will contribute to the break- up of Isis by quickly spreading internal criticism. Isis has been adept at using social media such as Twitter along with newer, smaller platforms – Ask.fm and Kik, Quitter and Diaspora – to broadcast its message. Barrett notes the group is “intolerant of any opposition or divergence from its worldview, and has set up networks of informers and a heavy security apparatus, managed from the centre, to ensure that no challenge to its authority can grow”.

This system has meant that those living under Isis have “begun to see it as merely a new form of oppression”, with residents recently revealing that conditions inside Mosul, the largest city under Isis control, have deteriorated. Barrett also says that the “hostility of Islamic State to individuality has also driven away many members of the professional classes, leaving hospitals without staff or medicines and schools without teachers”.

It is this, the administration of territory, that will play a key role in the longevity of Isis, emphasising the importance of agencies such as Isis’s Islamic administration of public services including electricity, sanitation and water.

However, the complexity of the challenge facing Isis is encapsulated by the fact that, while the seizure of grain stores has lowered prices, and keep bakeries running, many farmers have fled, meaning the crop for next year remains unplanted. The report adds that the amount of money required to run the caliphate “may equate to those of the Iraqi government before Islamic State took control”.

ISIS FACTS

■ Around six million people are currently living under its rule.

■ Employs between 20,000 and 31,500 fighters, according to US intelligence estimates, and another army of administrators to keep Isis functioning.

■ Controls land in Iraq that accounts for 40% of national wheat production.

■ Its latest annual report, which covers the 12 months to November 2013, demonstrates an increase in capability, with more than 9,000 military operations recorded, many indicating a high level of tactical flexibility.

■ Reportedly pays fighters between $200 and $600 a month.

■ Administration employees paid around $300 rising to $2,000 for senior managers.

■ More than four million mentions of the English acronym Isis between 17 September and 17 October 2014 on Twitter; the Arabic acronym mentioned 1.9m times over the same period.