DAVID Cameron targeted the growing UKIP threat in the Tory heartlands with an audacious £1.6bn tax boost for higher earners today (Wednesday).

The prime minister sparked a surprise by pledging to hike the threshold for paying 40p income tax from £41,865 to £50,000 by 2020, if he wins the general election.

The move, greeted with wild applause by activists in Birmingham, will be worth £1,313 to people earning between £50,000 and £100,000, before inflation is taken into account.

It is aimed squarely at the South, where 20 per cent of taxpayers pay the 40p rate - far more than in the North-East (11.3 per cent) and Yorkshire (12 per cent) - and where UKIP is on the march.

In a passionate conference speech, Mr Cameron also announced that the threshold for basic-rate taxpayers would rise to £12,500 by 2020, at a cost of a further £5.6bn

And he said: “If you work 30 hours a week on minimum wage, you will pay no income tax at all. Nothing. Zero. Zilch.

“Lower taxes for our hardworking people - that's what I call a Britain that everyone is proud to call home.”

The prime minister explained the help for 40p taxpayers on the basis that “teachers and police officers” had been dragged into a rate meant only for “the most well-off”.

But the move is controversial, coming just days after the Tories hit poorer people with a two-year freeze to benefits and tax credits – with £9bn of further welfare cuts still to be found.

Furthermore, although raising the basic-rate threshold lifts some lower-paid people out of paying income tax, most of the benefit flows to higher earners.

In his speech, Mr Cameron acknowledged the need to find a further £25bn of overall spending cuts in the next two years, but simply said: “It's doable.”

Labour was quick to attack “pie in the sky promises of tax cuts in six years' time when David Cameron cannot tell us where the money is coming from”.

Shadow Chancellor Ed Balls said: “How will they pay for it? Will they raise VAT on families and pensioners again?”

But Stockton South Conservative MP James Wharton tweeted his approval, saying: “PM absolutely right to pledge first £12.5k tax free and raise the level at which the top rate kicks in to £50k. Will help a lot of people.”

In the speech, Mr Cameron also promised:

* 100,000 new starter homes - available to first-time buyers at a 20 per cent discount and not “wealthy foreigners”.

* To protect the NHS budget for the next Parliament.

* To scrap the Human Rights Act – with more details expected tomorrow. (Fri)

* A place on the National Citizenship Service scheme for every teenager.

* To deliver ‘English votes for English laws’ – in the wake of greater devolution to Scotland.

Mr Cameron also sought to convince voters they must make a straight choice between Conservatives or Labour, saying: “Me in Downing Street, or Ed Miliband in Downing Street?”

And, addressing the UKIP threat directly, he added: “If you vote UKIP, that's really a vote for Labour.

“Here's a thought - on May7, you could go to bed with Nigel Farage, and wake up with Ed Miliband.”