Plebgate affair could have been a "gigantic conspiracy," No10 believed Downing Street considered the possibility that chief whip Andrew Mitchell may have been the victim of a "gigantic conspiracy" over the "plebgate" affair, David Cameron's most senior civil servant said yesterday. But Cabinet Secretary Sir Jeremy Heywood, who was asked by the Prime Minister to look at CCTV footage of an incident in which Mr Mitchell is alleged to have sworn at officers guarding Downing Street, said that after his "little review" it was decided not to look further into the affair but to "let matters rest." The Cabinet Secretary came under fire during an hour-long hearing at the House of Commons for failing to establish exactly what had happened, with one senior MP telling him the shortcomings of his inquiry had cost a Cabinet minister his job. Mr Cameron refused to sack the chief whip, who allegedly called the police "plebs," but Mr Mitchell eventually resigned in October after weeks of ­controversy. Sir Jeremy told the Commons public administration committee that Mr Cameron asked him to carry out a "little review" to establish whether allegations made in two e-mails from someone who claimed to have witnessed the argument were accurate. The allegations, that Mr Mitchell called officers "plebs" after they refused to let him cycle through the Downing Street gates, echoed the account recorded in the police log by the officers themselves, which the former chief whip denies. After viewing CCTV footage, Sir Jeremy told Mr Cameron that it disclosed "inaccuracies and inconsistencies" in the e-mails, which meant that they could not be relied upon when deciding whether Mr Mitchell should be sacked. The Cabinet Secretary said that he did not ask to view the log or speak to the officers involved and did not look into the issue of whether the word "pleb" had been used. He said he did not know at the time that the e-mails had been sent by a serving police officer. A Metropolitan Police investigation was later launched following allegations that the e-mails came from a member of the force. Asked whether it occurred to anyone in Downing Street that there could have been "massive fabrication" to undermine the chief whip, Sir Jeremy replied: "We accepted there were unanswered questions, including the possibility of a gigantic conspiracy, or a small conspiracy. Those were unanswered questions. But we decided, on balance, to let matters rest as they were, decided to stick by Andrew Mitchell, keep him in post and move on. Sir Jeremy insisted that he did a "competent" job and that his investigation came to the correct conclusion. Committee chairman Bernard Jenkin told him Number 10 had "some very big lessons to learn." "We keep coming across this point that your remit was very narrow and you only did what the Prime Minister asked you to do," said Mr Jenkin. Don't you think that is a fundamental problem with this investigation, that you weren't asked to get to the bottom of it, you didn't feel it was your obligation to get to the bottom of it and by failing to get to the bottom of it, the government lost its chief whip? Sir Jeremy responded: "I don't really accept any of those points. The fact of the matter is that my job as Cabinet Secretary in this regard is to carry out these reviews that the Prime Minister asks me to do, and that is what I did in good faith. Downing Street gave Sir Jeremy its backing following the hearing.