Blair, 50, felt unwell on Wednesday evening, just five weeks after he suffered a heart scare.
A family doctor attended him but was sufficiently concerned to call in a specialist, the premier's spokesman told reporters in London on Thursday.
"He is fine," the spokesman said. "The problem appeared to pass pretty quickly.
"The prime minister was experiencing stomach pains. A GP (general practice family doctor) came and he thought it would be useful to have another opinion," he said.
Blair's spokesman said the prime minister chaired his Cabinet meeting as usual early on Thursday and was heading for Wales later in the day for a Labour Party policy forum.
"Whatever it was didn't require any treatment," he said, but admitted that a doctor would check on Blair again at some point on Thursday.
On 19 October Blair received electro-cardiac treatment at a London hospital after being admitted with heart palpitations.
Blair took over the Labour Party leadership when his predecessor John Smith died of a heart attack in 1994. He won the 1997 election to take office as Britain's youngest prime minister since 1812.