AFP

Two Pakistani men apparently discussing girls online were really planning a massive Al Qaeda terror attack on Britain, a senior British intelligence officer has testified in court.

The officer gave his testimony to the Special Immigration Appeals Commission as part of a deportation hearing following the arrest of five Pakistani men in anti-terror raids in Manchester and Liverpool in April last year.

Three of the men have been deported to Pakistan and are appealing to be allowed to return to Britain. The two others are fighting their deportation order - apparently backed by Pakistani government ministers.

The officer, identified only as ZR, told the court on Tuesday the five men planned to stage an attack between April 15 and 20 last year - just days after their arrest - in a plot that would inflict "mass casualties".

He told the hearing that that the alleged plot ringleader Abid Naseer exchanged coded emails with an Al Qaeda operative called Sohaib, using girls' names in order to conceal their identities.

"On the face of it the emails are designed to look, without knowledge of the surrounding context, they're designed to look like correspondence between two people about girls," he said, according to the Daily Mail.

"I don't assess this to be two young men simply talking about girls. I think it's two people discussing attack planning on behalf of Al Qaeda," he added.

Police say Naseer and Ahmed Faraz Khan were part of a major plot and constitute a risk to security, but that it did not have enough evidence to charge them with criminal offences.

Naseer's lawyer Jeol Bannathan told ZR: "You know, don't you, that in Pakistan in public, senior figures in the Pakistani government have been saying 'these boys are innocent, they should be allowed to carry on with their studies'."

ZR replied: "The claims that these individuals are not involved in terrorism are wrong."

The British intelligence officer compared the alleged plot to the July 7, 2005 attacks in London and a trans-Atlantic airline bomb plot of 2006, but refused to reveal details of the similarities in open court.

"The overarching similar fashion was that they were all planning a terrorist attack in the UK under the direction of Al Qaeda and these were aiming for mass casualties."

He added: "I'm comfortable that there are similarities between those plots and I'm comfortable to talk about that in closed session."

MI5, the British spy agency, claims that Naseer and Sohaib used names of girlfriends to refer to different explosive materials needed for homemade bombs.

They also mentioned a wedding, which was in fact the alleged attack, the security service says.