"To the European people ... you only have a few more days to accept bin Ladin's truce or you will only have yourselves to blame," said the statement purported to be from the Abu Hafs al-Masri Brigades which claimed responsibility for the 11 March train bombings in Spain.
In an immediate reaction, Britain and Germany dismissed the threat as not credible.
"They claimed the Madrid bombings and it clearly wasn't them. It's the sort of thing they say all the time. They're just repeating the same old bile," said a British security source.
The source said Britain was on high alert for attacks from Islamic rebels, but did not consider Friday's letter to be an important development.
The Abu Hafs al-Masri Brigades
took credit for the Madrid blasts
"The general threat is taken seriously. But what these people say doesn't add anything significantly to that," he said.
Germany, which is maintaining a very high level of security in the face of a general threat from Islamic rebel groups, reacted by saying it did not consider the threat particularly credible.
"This is a sinister organisation which has already laid claim to many things, including power failures in America. That means that these statements should be taken with great caution and are probably not particularly credible," an Interior Ministry spokesman told a regular news conference.
Attacks to continue
Al-Qaida leader bin Ladin, in an audiotape on 15 April, extended a truce to Europeans if they withdrew troops from Muslim nations. He said the offer not to attack Europe would last three months.
It was not immediately possible to verify the authenticity of the new statement, parts of which were published on Friday by London-based newspapers Asharq al-Awsat and al-Hayat.
The dailies said the letter was dated 1 July but did not say how they obtained it. "Muslims in the West should depart to Muslim states if they can," the letter said.
Security will remain tight despite
Britain's dismissal of the threat
"Those who cannot should take precautions and live in Muslim areas, have enough food to last a month, find ways to protect themselves and their families, leave enough money in the house to last one month or longer and to pray a lot and put their fate in God's hands," it added.
The statement said attacks would continue until the United States freed Muslim prisoners, ended its war on Islam and until "all Muslim land, including Jerusalem and Kashmir, is cleansed of the stain of Jews, Americans and Hindus".
Several European states had rejected the truce offer, which excluded the US and Israel.
Abu Hafs al-Masri Brigades, which aligns itself to al-Qaida, is named after Muhammad Atif, also known as Abu Hafs, a bin Ladin aide killed in the US-led war in Afghanistan in 2001.
"Muslims in the West should depart to Muslim states if they can"
statement purportedly from the
Abu Hafs al-Masri Brigades
The group said those involved in "dialogue of civilisations" had little time to convince Europe to accept the truce.
"The race now is between you, time and European governments which refused to stop their attacks against Muslims. So do not blame us for what will happen, and we apologise to you in advance if you are among those killed."
The group also said it planned attacks in Yemen to "drag America into a third quagmire after Iraq and Afghanistan".
Yemen is bin Ladin's ancestral homeland and has cooperated closely with the United States to crush his network.