Allies of animal charity boss Pen Farthing and a Labour MP have cast doubt on Downing Street claims about a letter by Boris Johnson’s parliamentary aide Trudy Harrison giving him permission to be evacuated from Kabul in August. No 10 said Harrison was “acting in her capacity as a constituency MP” when she wrote the letter – as it continued to insist that Boris Johnson had not ordered the rescue of Farthing and his cats and dogs ahead of desperate Afghans. But Dominic Dyer, an animal rights campaigner lobbying to help Farthing, said on Wednesday that neither he nor Farthing were constituents of Harrison but that she had become involved in their campaign after some of her constituents had raised it. “From my perspective, Trudy was in contact with the PM. I understood the PM was committed to the operation and seeing it happened,” said Dyer, who had been intensively lobbying government over Farthing in late August. “Pen needed something he could hold up at Kabul airport. To be fair to Trudy, she did a good job, and said, ‘I’ll see what I can do.’ I don’t know if she took it to Johnson, but I can’t believe he wasn’t aware it was written. I can’t believe he wasn’t aware we’ve tied up the loose ends.” Harrison’s letter, which emerged on Tuesday night, was dated 25 August and signed by her in her capacity as MP for Copeland in Cumbria and as “parliamentary private secretary to the prime minister”. It spelled out that the Foreign Office, Home Office and the Ministry of Defence – which had made last-minute objections – had cleared the way for Farthing and staff at his Nowzad charity to be evacuated from Kabul’s airport. “You are therefore authorised to proceed,” she wrote. Dyer said he believed the letter was “not the letter of an ordinary constituency MP”. He said “it was sought because Pen did not want to rely on emails from the Foreign Office. It was there to show he and his staff had the legal right to leave.” Harrison, however, said the letter, which had a Commons header, “was sent on parliamentary paper to confirm appropriate security clearance had been provided”. She added: “Whilst I was indeed the PM’s PPS, this was not a matter for him. It was a routine task in response to many Copeland constituents’ requests to assist.” Chris Bryant, a Labour member of the foreign affairs select committee, who had raised the issue earlier this week, said he was sceptical of No 10 and Harrison’s explanations. “I just don’t buy it. It all seems far too convenient and murky. It still seems clear to me that the prime minister effectively if not directly ordered this evacuation. He just didn’t want his sticky fingerprints all over it.” Conservative MPs contacted by the Guardian said it would have been unusual to deliberately reference her role as Johnson’s aide in the Commons if she did not intend to signal it had the backing of the prime minister. Party sources said there was unease at what they believed was an approach that amounted to the prioritising of “animals ahead of Afghans” in the final days in Kabul. Farthing and over 150 cats and dogs were rescued in one of the last flights out of Kabul, but last-minute delays at the airport meant that over 60 staff at the Nowzad charity and dependants had to cross the border to Pakistan before they could come to the UK. “This was always a wider humanitarian mission,” Dyer said.