Blair told the German newspaper Die Zeit that the exodus could happen if too much power was transferred to Brussels, the Daily Telegraph reported.
Blair said he was "deeply worried that Britain could decide by referendum to leave the whole process" if the eurozone debt crisis led to "powerful political change of the EU."
If Britain granted the EU more powers, Blair said, "Britain must play a strong role in this. Because we need a balance between European institutions and the nation states. "
A majority of the British would reject Britain's membership in the EU in a putative referendum, polls have found.
Prime Minister David Cameron said in July he would resist pulling Britain out of the EU because such a move could damage British interests.
Cameron has often taken a hard line with the EU. In December, he turned down a new fiscal agreement.
Those who support leaving the eurozone say Britain could save billions of pounds in membership dues, restore the country's sovereignty and remove restraints on the British economy.