The Conservatives are planning a ‘kill Ukip’ campaign in the Kent constituency where Nigel Farage is standing for Parliament after he vowed to quit as party leader if he loses

Tory HQ is planning a ‘kill Ukip’ campaign in the Kent constituency where Nigel Farage is standing for Parliament after he vowed to quit as party leader if he loses.

Mr Farage declared yesterday that it would be ‘curtains’ for him if he fails to win the Ukip target seat of South Thanet.

He conceded it would be impossible for him to continue to dictate Ukip policy without a seat in the House of Commons.

Ukip is aiming to win around half a dozen seats, but Mr Farage appears to face a tougher fight than Douglas Carswell, the Tory defector who won a big majority when he stood for Ukip in a by-election.

Polls suggest he is in a three-way battle with Labour and the Conservatives. The Tories hold the seat, while Labour previously held it with slightly different borders.

Conservative strategists believe Mr Farage has made a significant blunder by tying his fortunes in South Thanet to his future as party leader.

‘This is a great opportunity,’ said one. ‘We can now make this a decapitation campaign. If voters who have previously backed other parties want to stop Ukip, they can vote for us and cut the head off the snake.’

The strategist said that if Mr Farage stood down after losing the seat, Mr Carswell was likely to take over and ‘that would be the end of Ukip’.

Other candidates to succeed the Ukip leader include communities spokesman Suzanne Evans and former journalist Patrick O’Flynn, both Euro MPs.

Mr Farage appears at odds over several areas of policy with key members of his team, Mr Carswell, who has expressed concern about the ‘ugly nativism’ of some Ukip supporters and spoken in favour of immigration.

The Clacton MP is also seen as lacking in the broad popular appeal of Mr Farage by Tory campaign chiefs.

A Survation poll commissioned by a Ukip donor in South Thanet found Mr Farage to be ahead, but one previous survey had him neck and neck with the Tories and another put the Tories ahead.

If voters who have previously backed other parties want to stop Ukip, they can vote for us and cut the head off the snake

In an extract from a new book by the Ukip leader, The Purple Revolution, serialised in the Daily Telegraph, Mr Farage made clear he would quit as party leader if he fails to become an MP.

‘It is frankly just not credible for me to continue to lead the party without a Westminster seat,’ he wrote.

‘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.’

Yesterday Ukip won a boost when broadcasting regulator Ofcom awarded it a second party election broadcast on commercial TV and radio on the basis that it is now a ‘major party’.

David Cameron's strategists hope that defeating Mr Farage would 'cut the head off the snake' of Ukip

The bodys guaranteed Ukip a minimum of two broadcasts in England and Wales on ITV, Channel 4 and Channel 5, alongside the Conservatives, Labour and Liberal Democrats.

But it said the Greens had not demonstrated sufficient electoral support to justify a similar elevation in status.