The Ministry of Defence is trying to block all internet access to the whistleblowing site Wikileaks from thousands of its own computers after discovering that dissidents have been using it to leak copies of British military manuals.

Newly obtained MoD emails reveal alarm over the discovery that Wikileaks is freely publishing manuals that are used by patrols in Iraq.

One email says: "There are thousands of things on here, I literally mean thousands. Not just UK MoD but other places as well. Everything I clicked on to do with MoD was 'restricted' ... It is huge."

Dismay was sparked by the discovery that Wikileaks had posted copies of a 2007 manual on UK tactics for Iraq and Afghanistan, and a handbook on Istar (intelligence surveillance, target acquisition and reconnaissance) on its website.

The Iraq "stability operations" manual contains details on how to organise roadblocks, conduct searches and tackle suicide bombers, advising: "Immediate incapacitation of the suicide bomber, using lethal force, is likely to be the only means of stopping him."

The manual contains "cultural awareness" advice on Arab and Afghan etiquette, warning soldiers that it is "very rude" to make the thumbs-up sign in Afghanistan. It advises: "Feel free to return a hug or kiss on the cheek, initiated by an Iraqi man. This is a sign of friendship, not homosexuality."

"Restricted" is the lowest level of security classification, but MoD officials feared that terminals at some of Britain's 1,100 military locations were being used by disaffected soldiers to email copies of the documents.

After an "urgent" investigation, one unnamed official warned that it "might be quite tricky to block" because Wikileaks has a variety of domain names.

The MoD's directorate of information, headed by John Taylor, nevertheless went ahead and instructed BT, which has a private contract to manage the military's landlines, to block access to Wikileaks and one of their main servers in Sweden.

A Wikileaks source said yesterday: "Wikileaks has many alternative names. MoD site blocking has not prevented dissident MoD staff providing us with material exposing the UK involvement in Iraq and elsewhere."

Asked why it had blocked access to Wikileaks, an MoD spokesman told the Guardian: "MoD staff are unable to access Wikileaks because there is no compelling business reason to have the facility."

The MoD released some of its emails following a freedom of information request.