A Labour donor worth more than £1 billion has disclosed that he will help to fund a general election campaign if Gordon Brown chooses to go to the country this autumn.

Lord Paul, a philanthropist who helped to save London Zoo, smoothed the way to an early poll by saying that he would boost election coffers by “as much as I can afford”.

The steel billionaire, who hosted a gathering attended by many senior Brownite supporters at the zoo shortly after Mr Brown’s succession, gave a welcome boost to Labour chiefs worried by the party’s £20 million debts. Other donors are believed to be waiting in the wings if Mr Brown goes against his earlier inclination and calls an election within weeks.

Party sources say that they are confident of raising enough for an early election. Mr Brown is believed to be unlikely to conduct a lavish campaign and may make a virtue of keeping below the spending limit of £20 million. It is understood Labour has already approached advertising agencies over the handling of its campaign. At least three, including Beattie McGuinness Bungay, run by Trevor Beattie who was involved in the Blair election victories, are thought to be in the running. Mr Brown has also recently appointed a new director of election resources, Jon Mendelsohn.

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It has also emerged that Mr Brown has spent £39 billion in his first seven weeks at No 10, by preannouncing the release of money from the Comprehensive Spending Review, due in October. Defence has been promised £7 billion extra, the railways £15 billion, housing £8 billion and preschool education £4 billion.

Lord Paul has been a friend of Mr Brown and his wife, Sarah, for many years. He is chairman of the 2012 Olympic Delivery Authority. He is nondomiciled in Britain for tax purposes but insists that he pays big sums in tax to the Treasury each year.

In The Sunday TimesRich List 2007, the wealth of Lord Paul and his family was put at £1.5 billion. In a Channel 4 interview he said: “All I know is that if there is an election, whatever I can pay I will pay. I am a believer in Gordon Brown and I think he’s the best person in the country.”

Asked how much he would pay he replied: “As much as I can afford.” Sir Ronald Cohen, a venture capital tycoon, is another big financial backer of Mr Brown, and there has been no suggestion that Lord Sainsbury of Turville, who gave big sums to Labour under Mr Blair, will not be equally generous under Mr Brown.

Party strategists are playing down suggestions that the SNP’s continued strength in Scotland will deter Mr Brown. Analysis of nationwide polls shows that Labour’s support in Scotland is similar to that at the 2005 election. Mr Brown has six weeks before Labour’s conference to decide whether to go for an early election. If he sticks to his original intention of a poll next summer he is likely to signal his intention in September. Were he to go for an autumn poll, the Commons would have to be recalled in October, if only to be dissolved for the election.

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