A minister has rounded on the Prime Minister's father after Stanley Johnson personally intervened to save Geronimo the alpaca from being put down.

       Geronimo, who is six, has twice tested positive for bovine tuberculosis and is set to be euthanised. Helen Macdonald, the alpaca's owner, has recently lost her final appeal at the High Court in London to save her beloved animal but argues the bovine tuberculosis tests have been returning false positives.

       Mr Johnson senior has written to his son and George Eustice, the Environment Secretary, urging them to stop “their absurd murderous errand”.

       Writing for The Sun, he claimed the case was “not an -animal welfare story. It is also the story of one woman’s fight against a system”.

       He told Mr Eustice: “Tear up that threatening letter, Minister. Write another one, a nice friendly one. Give Geronimo a new test.”

       But Kwasi Kwarteng, the Business Secretary, showed no sign of wavering.

       He told Sky News: “I am not sure where Stanley is coming from on this. My understanding is the alpaca tested positive for TB and the rules are quite clear.

       “Bovine TB is really damaging.”

       There has been an outpouring of support from the public to save the condemned alpaca, with more than 90,000 people signing a petition calling on the Prime Minister to stop the killing.

       However, Mr Kwarteng insisted the row was not a “PR disaster”, saying it was “an August story”.

       He added: “People have invested emotionally in a story of the animal but there is a policy and there is no reason why the policy shouldn't be stuck to.”

       It comes as animal rights protesters and a number of alpacas are due to descend on Whitehall today.

       Campaigners, with a group of the animals who have been trained to walk with people and are comfortable around crowds, are expected to gather outside Defra's Westminster headquarters at 2pm today before marching to the gates of Downing Street.