Gordon Brown yesterday invoked God to attack the Conservatives and pledged to go into the closing stages of the election campaign with renewed energy.

In a highly personal interview, with just 10 days of campaigning left before the 6 May election, and Labour third place in the polls, the British prime minister said he drew strength from his upbringing where he had to "fight for everything" and insisted the election remains "wide open".

Speaking of his anger at Conservative leader David Cameron's plans to lift all estates up to £1m out of inheritance tax, Brown said: "How can it be a priority to give to people who have already got so much? It's not 'God helps people who help themselves', it's 'God helps people whom he has already helped'. That's what [the Conservatives'] motto is."

The quote "God helps those who help themselves" is attributed to US president Benjamin Franklin.

In his first speech to his party's conference as Labour leader in 2007, Brown, the son of a Church of Scotland minister, quoted from the Parable of the Talents in the Bible to illustrate his mission for social mobility.

But this is the first time Brown has used God so directly to attack an opponent, and wading into the controversial area of religion for political purposes could be seen as risky.

In his interview, the Prime Minister gave an impassioned defence of his character and determination to fight for an outright victory, despite dismal poll ratings.

He also failed to rule out a scenario being discussed among senior Labour and Lib Dem figures where, if the Lib Dem leader Nick Clegg refused to work with him in a hung Parliament, he would hand the Labour leadership to Alan Johnson or David Miliband to form a "progressive coalition" with the Lib Dems.

The latest poll puts Labour on 28 per cent, up three points on last Wednesday, but in third place behind the Lib Dems on 29 per cent, and six points adrift of the Conservatives' 34 per cent. The poll suggests Clegg's insurgency is continuing, despite being checked slightly by Cameron's improved performance in the second televised leaders' debate last Thursday.

The survey suggests the election will result in a hung parliament.

Brown, speaking en route to the marginal territory of the East Midlands yesterday, said: "People know that the fight is on. They know that the election is wide open, they know that the closed book that people expected it to be a few weeks ago is not where we are."

In a swipe at the popularity of Clegg following his performance in the first TV debate, Brown added: "You start a campaign by people focusing on novelty, and on personality and style, but you end the campaign where people actually think: 'Here I am, a voter, what's going to happen to my job, what's going to happen to my family, my police, my school?'"

He stressed: "I am working for a majority Labour government."

In a veiled reference to both those in the Labour party who tried to topple him and to the privileged upbringings of Cameron and other senior Tories, Brown said: "I am a fighter, I always have been. I come from a background where you've got to fight for everything. Nothing ever comes easy by just good fortune. You've got to fight for everything you do. When things are tough, you've got to get up in the morning and show some resilience and determination and fight through it."

He attacked the Tories' policy of cutting tax credits for those on middle incomes while giving inheritance tax cuts of £200,000 for the 3,000 richest estates, "most of whom the leaders of the Conservative Party know by name".

Displaying a flash of anger, he added: "How can you tolerate that level of unfairness when you are dealing with young people who are unemployed, dealing with mothers who are going to lose child tax credits, dealing with urgent needs care for the elderly?"

The emergence of the Lib Dems in the race, reducing the Tories' lead, has paradoxically benefited Labour because it makes an outright Conservative victory less likely. Yet there has been a change in Labour strategy from a week ago, when Brown was making overtures to Clegg to pave the way for a "progressive alliance" to keep the Tories out of power.

There has also been speculation of a rift between Peter Mandelson, who is running the campaign, and Ed Balls, Brown's closest ally, over strategy. This weekend, the Labour approach to the Lib Dems hardened. Yesterday, Labour's David Miliband claimed that Clegg's "anti-politics" message was no basis for government.

Brown said there was "some common ground" with the Lib Dems on political reform but there were fundamental disagreements over the Lib Dem plans to not replace Trident, scrap child trust funds and tax credits for higher earners.

"I am not going to talk about understandings, what happens after the election, I am going to put my manifesto for the election," the PM said.

Asked if he felt he was bigger than his party, he replied: "No one is bigger than the party: no one, and certainly not me. [But] if you want to write about what happens after the election, you've got plenty of time to do it after the election is finished."