A masked al-Qaida fighter in Syria described as a former Arsenal footballer has been identified as a Portuguese citizen, it can be disclosed.

The man appeared in a video released online two weeks ago urging others to join the jihadists, using a false name and speaking in a heavy accent.

Rumours spread in the media and across the internet that the man was Lassana Diarra, 29, who played international football for France and for clubs including Arsenal, Chelsea, Portsmouth. Diarra was forced to deny he had ever set foot in the war-torn country.

The video was posted in an account linked to the Islamic State of Iraq and Syria (Isis) which is associated with al-Qaida.

The player was using the nickname Abu Isa Andaluzi, and the caption said: "An ex-football player (Arsenal of London), who left everything for jihad."

Text next to the video read: "He... played for Arsenal in London, and left football, money and the European way of life to make the path of Allah."

According to FiSyria.com, the site that posted the video, the man moved to London from his native country to play for Arsenal after growing up alongside a world-famous footballer.

British and foreign security services performed analysis on his accent and learned that he is probably Portuguese, sources have now disclosed.

They believe the man is an individual called Celso Rodrigues Da Costa who had been living in Leyton, east London with his two brothers.

It is unclear whether he had trials for Arsenal or ever had any association with the club.

Sources at Arsenal say they do not believe he was a full-time player but he may have attended coaching sessions, when potential players are assessed before signing for the club at the age of 16.

The video has been shown to coaches at the Arsenal academy who did not recognise the man's voice, but one source said: "All we have to go on is a muffled accent and a set of eyes."

A spokesman for the club said: "We do not recognise the individual from the published clips and we don't have any record of a Mr Celso Rodrigues Da Costa representing the club at any level."

At least two other Portuguese men based in London are also thought to be in Syria, one of them in the besieged city of Aleppo.

MI5 is concerned that jihadists from across Europe are collecting in Syria and that they may pool knowledge to launch attacks on the west.

A group from west London featured in photographs alongside a former Dutch soldier who has been training jihadists in Syria.

Charles Farr, the head of the Office for Security and Counter-Terrorism, said this week that Syria was the "biggest challenge" facing the security services and described the situation as "extremely hazardous."

Shiraz Maher of the International Centre for the Study of Radicalisation (ICSR) at King's College, London, said: "This demonstrates just how appealing the Syrian conflict has become to a broad and diverse range of people. There are now people from 74 different countries participating in this jihad, and they come from all walks of life."

In the video posted last week, the man believed to be Da Costa was wearing a balaclava and a relatively smart, green quilted jacket as he sat beside a lake, brandishing an AK-47.

He was sending a message to the people of the Ukraine, apparently in the belief that Muslims in the Ukraine or Crimea might want to join the fight in Syria.

The video had been subtitled in Cyrillic and the man told his audience: "My advice to you first of all is that we are in need of all types of help from those who can help in fighting the enemy. Welcome, come and find us and from those who think that they cannot fight they should also come and join us for example because it maybe that they can help us in something else, for example help with medicine , help financially, help with advice, help with any other qualities and any other skills they might have, and give and pass on this knowledge, and we will take whatever is beneficial and that way they will participate in jihad."

In a second section of the eight minute video, he encouraged women to join the fighters: "Even the enemies of Islam are paying us jizya [tax], this is something very important to mention, you can see the sincerity of the mujahideen [fighters]. If you have families in kaffir [heathen] countries, what will happen most probably is you don't have control of your children. Some countries you must put your children in kaffir schools and who is going to teach your children, it's going to be maybe a gay, maybe a drug dealer, maybe a paedophile. It's very important for you to protect your children from these animals, these dirty people."

A player with the under-17 German national squad was killed fighting in Syria in October. Burak Karan, 26, died after a bomb was dropped from a Syrian Air Force jet on the village of Azaz, near the Turkish border.

An image of him clutching a Kalashnikov assault rifle appeared in tribute to him on social networking sites.

In 2003, Nizar Trabelsi, a former professional football player with Fortuna Düsseldorf and Wuppertal in Germany was sentenced to 10 years in jail for his association with al-Qaeda and for plotting to attack US targets including American soldiers stationed at the Belgian airbase Kleine Brogel.

In November, 2001 author Adam Robinson claimed in his biography 'Bin Laden: Behind The Mask of Terror' that Osama bin Laden had attended four matches at Arsenal's ground in Highbury when he visited London in the 1990s.

He said that bin Laden witnessed Arsenal's run to the European Cup Winners' Cup final in the 1993-1994 season and was so enthralled by George Graham's side that he bought his eldest son, Abdullah, an Ian Wright replica shirt.

Arsenal enforced an immediate ban, saying: "We've seen the reports in the papers. Clearly he wouldn't be welcome at Highbury in the future."