Yesterday police in Tanzania reported they had arrested a man named locally as Ahsan Ali Iqbal, 19, said to have a British passport, who is suspected of involvement in unspecified "terrorism activities" in the UK.

However there is growing alarm at the numbers now travelling to Syria, and a British-led European initiative is being launched to counter the threat. Measures being considered include making it illegal to travel to take part in jihad, banning specific organisations, freezing bank accounts, deporting Muslim preachers, and even taking away "social benefits".

The Home Secretary, Theresa May, is leading discussions between a group of European Union member states, understood to include France, Denmark, Belgium, Spain, Sweden, the Netherlands and Ireland, to look at ways of combating the threat posed by so-called "jihad tourism". The group was formed after a meeting last month of senior ministers in Luxembourg, where the issue of foreign fighters was made an EU "key priority".

Spain's Secretary of State for Security, Francisco Martinez Vazquez, said Europe was witnessing a new kind of terrorism "characterised by self-radicalisation and self-training mainly through the internet, which is absolutely different to what we knew before". The minister, who was at the talks with Mrs May, said that a "common approach" is needed across Europe – with new laws to help deal with the threat.

He added: "Following a global jihad is not yet a crime in most European countries. This means we don't have the proper tools to fight against it." Mr Martinez Vazquez said it should be a "criminal offence" for people to use the internet to become "self-radicalised".

Charles Farr, director general of the UK Office for Security and Counter-Terrorism, warned last week that the civil war in Syria represents the biggest terrorist threat to Britain and Europe for a decade. "Syria is a very profound game changer and the significance of it is still emerging," Mr Farr said.

There have been a number of UK arrests in the past year of people going to or from Syria. At least two young Britons have been killed fighting with the rebels – part of a global death toll that includes people from 29 different countries. Yet it is those who survive and return who are causing the greatest concern.

The Foreign Office minister Baroness Warsi said the UK had gathered "credible evidence that up to 100 young British people, or people connected to the United Kingdom, are out there fighting" – although experts claim the real figure is higher.

Firas Abi Ali, a Middle East expert at th analysis company HIS, warned that veterans from Syria would return with "expertise and intent" and so the "capability to launch attacks in the UK will probably increase".

"I think this is only going to get worse as time goes by," he said. "It would be very difficult for any of these groups to mount a sustained campaign against the United Kingdom... but a single spectacular [attack] will become a lot easier to achieve as a result of Syria."

Islamic scholars throughout the Middle East last month called on Muslims to go on jihad in Syria. Aaron Zelin, the Richard Borow fellow at the Washington Institute, said: "I think it is getting worse, because of the calls for jihad by mainstream clerics." There will be many appeals made about Syria during Ramadan, something that could result in a "large number of individuals in the Arab world as well as in western Europe and in Britain, leaving for Syria after Ramadan."