More than 2,000 postal workers were tonight awaiting the results of anthrax tests after a colleague in Washington DC was diagnosed with the most serious form of the infection.

FBI detectives believe they have found the anthrax-laced letter which infected a worker at the New York Post newspaper, it was revealed today.

The anthrax strains surfacing with terrifying impact in Florida, New York and Washington are all similar, Bush administration said indicating a single source.