(Recasts, adds Cameron quotes)
By Peter Griffiths and Matt Falloon
LONDON, Jan 23 (Reuters) - British economic policy will not be influenced by speculators on financial markets, Prime Minister Gordon Brown said on Friday, as concerns intensify over sharp falls in sterling and the worsening state of the economy.
The speed at which the economy has gone into reverse -- it shrank at its fastest pace since 1980 in the three months to December -- and worries over how well the government's response is working have spooked financial markets in recent days.
Prominent international investor Jim Rogers said this week the pound was "finished" and people should avoid investing in Britain. Sterling hit a 23-year low against the dollar and a record low against the yen on Friday.
"If you think that we are going to build our policy around the comments of a few speculators who want to make money out of Britain, then you are very, very wrong indeed," Brown said.
For the time being, British policymakers seem content to let the pound drop in value because it provides a much-needed competitive edge for British exporters at a time of slowing global demand. It is hoped that will give the economy a boost.
Bank of England Governor Mervyn King reiterated that sentiment in a speech earlier this week.
But the fall in sterling is causing friction in Europe -- Britain's main trading partner -- and at home, where the opposition Conservatives have pounced on it as proof of global disapproval of the ruling Labour party's policies.
IMF WARNING
Conservative leader David Cameron has warned that Brown's decision to borrow billions to try to kick start the economy has left Britain "running the risk" of being forced to go to the International Monetary Fund for emergency help.
It was a Labour government that last took such action in 1976.
"I think it's right to warn about that, I think it's a responsible thing to do," Cameron told reporters. "I don't want the government to make it worse in their forthcoming March budget."
Brown described such comments as "irresponsible behaviour".
The prime minister was speaking before official data showed Britain had entered its first recession since the early 1990s. The economy shrank 1.5 percent in the three months to December and is expected to continue contracting for some time to come.
That deterioration has made the government's forecasts for a recovery in the second half of this year look extremely optimistic.
"It depends all the time on what is the level of international cooperation," Brown said. "Coming out of America in the next few weeks will be a major, major stimulus package that will help the rest of the world. Other countries in Europe are about to make decisions themselves about the future and I think that is very, very important as well."
Several major economies around the world have launched economic stimulus plans already but as activity slows sharply, expectations are growing that more action will be needed to stave off a severe global downturn.
"If for example China could create more domestic expansion itself, then the world would be going in harmony, going in tandem, to deal with ... what is a synchronised banking failure," Brown said. (Editing by Stephen Nisbet)