Police are on the alert after a fifth letter bomb, from suspected animal rights protesters, was received - this time by Chipperfield's circus.

The package was made safe by Army bomb disposal experts yesterday after it was delivered to the circus's winter headquarters at Heythrop, near Chipping Norton.

A Thames Valley Police spokesman said today: "This new letter bomb had the potential to cause serious injury to the recipient. At this stage we are linking it to the four devices which were received last week. This is a cowardly attack and it is an unacceptable form of protest."

Police believe the bomb is the same type as those posted to the homes of cat breeder Chris Brown, of Hillgrove Farm, Minster Lovell, and Prof Colin Blakemore, in Oxford, last week. Bombs were also sent to two workers at Hillgrove Farm. All four victims escaped unhurt after realising the packages were suspicious.

Earlier this year a group calling themselves the Provisional Animal Liberation Front sent Mr Brown a bomb threat.

Campaigners have staged demonstrations outside his farm over the last 12 months.

Prof Blakemore, Professor of Physiology at Oxford University, has been a regular target for extremists since 1987 after allegations about experiments on kittens as part of his work into the condition lazy eye.

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