President Obama has begun dampening expectations for his $787 billion (£548.1 billion) stimulus package with aides warning not to expect instant miracles.

"There will be signs of activity very quickly," David Axelrod, the White House senior adviser, said. "But it's going to take time for that to show up in the statistics. The President has said it's likely to get worse before it gets better."

White House spokesman Robert Gibbs was similarly cautious. "I don't think there's any doubt that we've seen this economy has gotten worse just in the last few months. The acceleration in job loss probably means that this economy is going to get worse before it gets better," he said. Obama is due tomorrow to sign the Bill passed last week by Congress.It was the first major legislative victory of his Presidency.

The stakes are high for the US and global economy and for Obama himself. He said last week that if he failed to heal the economy, he would be out of a job by 2012, when he faces re-election.

The White House hopes the package will save or create 3.5 million jobs but analysts are sceptical.

Mark Zandi, chief economist at Moody's Economy.com, said: "By my estimate, it will add 2-2.5 million jobs, more than would be the case without stimulus, by the end of 2010."