Nigel Farage has declared it will be “curtains” for him as UK Independence party leader if he fails to win his target parliamentary seat of South Thanet.

He said it would be impossible for him to sit and brief Ukip policy from the Westminster Arms pub without a seat in the House of Commons. Farage is by no means certain of winning the Kent constituency, which is a three-way battle between Ukip, Labour and the Conservatives. The Tories hold the seat but the incumbent MP, Laura Sandys, is stepping down, while Labour previously held the constituency with slightly different borders. A Survation poll commissioned by Ukip found Farage to be well ahead, with Labour second, but a previous survey had him neck and neck with the Tories. Ukip is unlikely to gain more than a handful of MPs at the election, but could come second in hundreds of seats across the country, setting it up for a potentially bigger breakthrough in 2020. Asked by the Guardian if he would still be around in Ukip at that point, Farage said: “I think if we get a good result on 7 May, I would like to be part of Ukip’s 2020 strategy. But if I fail on 7 May, we will have to look at this differently.” In an extract from his book, The Purple Revolution, serialised in the Daily Telegraph, Farage made it even clearer he would quit as party leader if he failed to become MP for South Thanet. “It is frankly just not credible for me to continue to lead the party without a Westminster seat,” he writes. “What credibility would Ukip have in the Commons if others had to enunciate party policy in parliament and the party leader was only allowed in as a guest? “Was I supposed to brief Ukip policy from the Westminster Arms? No – if I fail to win South Thanet, it is curtains for me. I will have to step down.” Ukip is most likely to hold Douglas Carswell’s seat of Clacton, which he won back last year after defecting from the Conservatives. The party is also hoping to retain Mark Reckless as its MP for Rochester and is also targeting Castle Point and Thurrock in Essex, Great Grimsby in Lincolnshire and several seats in South Yorkshire. Farage’s claims that he could not lead the party from outside the House of Commons suggests a successor if he loses in Thanet would have to be picked from among the crop of any new Ukip MPs.