Nigel Farage has vowed to quit as Ukip leader if he fails to become an MP at the general election.

In his book The Purple Revolution, he says it would be "curtains for me" if he lost the South Thanet seat.

Research suggests he is on course for victory, but the MEP will have to overturn a Conservative majority of nearly 17%.

He writes: “The battle to win South Thanet in the May general election may well prove to be my biggest yet. If we get between four and six seats, there is a real risk that I may not have one of them.”

He goes on: “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? Am 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.”

His revelation comes in a book which also reveals Rochester and Strood MP Mark Reckless was so terrified about being found out about secret talks with Nigel Farage over defecting that he donned disguises.

Mr Farage, whose book 'The Purple Revolution' is being serialised by The Daily Telegraph, reveals how he had held one meeting with the MP even before Douglas Carswell defected from the Conservatives to UKIP.

He also writes about how the MP was "bulllied" by Conservatives and attempts were made to smear him as an alcoholic.

He says Mr Reckless donned a baseball cap and dark glasses for one meeting.

He writes: "Mark Reckless came twice to see me at my home in Downe, Kent. No lunch, no wine, just pots of tea, and we talked.

"The first time he came was before Douglas joined, but the second was after.

"By then, the campaign to put the frighteners on any Tory looking to join us was intense, Mark turned up in dark glasses and a baseball cap so that the neighbours wouldn’t recognise him."

He was convinced he was being followed, most likely by someone at Conservative central office, Mr Farage writes.

He goes on to say that he suspected that everyone at Ukip HQ was having suspicions that their mobile phones were being tapped.

At a second meeting in Pimlico, Mr Reckless "arrived in disguise with a beanie-style woolly hat pulled down over his eyes. We were all slightly paranoid by now."

"If the Tories had believed he was on the brink of joining us, they could have thrown him out of the party, thereby discrediting him. I told Mark that he had one chance to really make a difference; he was extremely emotional."

During their discussions about whether he would defect, Mr Farage writes: "Mark and I had the same conversations as I’d had with Douglas months before. He had similar questions.

"Mark Reckless came twice to see me at my home in Downe, Kent. No lunch, no wine, just pots of tea, and we talked" - Nigel Farage

"Did we have enough money and people on the ground to run an election campaign and help him win Rochester back? He was putting his career on the line, after all.

"We knew even then that the Tories would throw everything at the Rochester by-election (and they later proved us right)."

While he had no evidence to back it up, he said Mark was regularly telephoned from Washington DC.

"Can I prove that the bullying was funded by Conservative central office? No. Do I suspect them? You bet. And it was disgusting."

Read the thoughts of our political editor Paul Francis on this and other election news in his blog here

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