The prime minister is facing an inquiry from the Office of National Statistics (ONS) after he revealed unemployment figures before they were officially released.
Speaking to a hostile TUC conference yesterday, Tony Blair - keen to defend the government's progress on jobs - apparently leaked the market sensitive statistics which he had received early.
He told delegates: "Tomorrow, I think, we will probably see for the first time in some months a fall again in unemployment."
The ONS data, which showed that the number of people claiming unemployment benefits had fallen by 3,900 in August, came as a surprise to analysts who predicted a rise of 4,000.
This took the number of people claiming jobseekers to 950,100 last month - down 3,900 on the previous month but up 80,800 on the year.
ONS statistics also show that despite this fall, the number of people out of work rose to its highest in nearly seven years - 1.7 million between May and July.
Mr Blair's comments caused the pound to climb after city traders saw them as a sign that the Bank of England's interest rates would rise again later this year.
Richard Alldritt of the Statistics Commission, which oversees the work of the ONS, said Mr Blair's comments looked "inconsistent with the code of practice" and warranted consideration.
"We have seen reports that the prime minister made remarks about the unemployment figures in advance of their publication," the ONS said in a statement.
"The national statistician is looking into the circumstances and has been in contact with the cabinet secretary. We will make a further statement shortly."
The Conservatives condemned the prime minister, with shadow chancellor George Osborne saying: "Tony Blair has made an irresponsible gaffe.
"Important economic data should not be blurted out at meetings with trade unionists. This proves why we need independent statistics that are not subject to ministerial spin. Last year Gordon Brown promised that would happen. But he has failed to deliver."
And Liberal Democrat Treasury spokesman Vince Cable argued that his remarks suggested "a certain naivety on economic policy matters".
But a spokesman for the prime minister said: "It's ridiculous to say that referring to statistics in a general way is a problem. He didn't give any detail."