The Senlis Council claimed that the insurgents controlled "vast swathes of unchallenged territory" and were gaining "more and more political legitimacy in the minds of the Afghan people".
It said that the Nato force in the country needed to be doubled to 80,000 front-line soldiers who should be allowed to pursue militants into Pakistan.
The 110-page report said that its "exclusive" research found the Taliban controlled 54 per cent of Afghanistan.
It calculated that Nato countries should contribute 2.3 soldiers per £500 million of their GDP to provide 71,000 soldiers, with 9,000 additional troops coming from Muslim nations.
If the plan were adopted, Britain would need to send 4,500 troops, significantly fewer than are deployed now.
British and American military leaders say the mission in Afghanistan has been hamstrung by Nato members refusing to send reinforcements or placing "caveats" on their duties when there.
There is no sign, despite pressure from the US and Britain, of any move within Nato to send reinforcements to Afghanistan.
The report said: "It is a sad indictment of the current state of Afghanistan that the question now appears to be not if the Taliban will return to Kabul, but when this will happen and in what form.
The oft-stated aim of reaching the city in 2008 appears more viable than ever and it is incumbent upon the international community to implement a new strategic paradigm for Afghanistan before time runs out".
The Ministry of Defence dismissed the report, saying its conclusion that the Taliban would take Kabul was not credible. "The Taliban does not pose a credible threat to the democratic Afghan government," a spokesman said.
The report coincides with a study from Oxfam for the House of Commons international development committee, which gives warning that the security situation in Afghanistan is deteriorating.
Oxfam said that Afghans faced "severe hardship comparable with sub-Saharan Africa" and that aid was not getting to the most needy.
The Senlis Council, a Brussels-based think tank that conducts research into drugs, military intervention and development policies in Afghanistan, earlier this month advocated the growth of opium poppies in the country for use as morphine abroad.
It said the project would weaken the Taliban by offering an option for farmers and starving the organisation of funding.