Prime Minister Tony Blair has been treated in hospital for a slipped disc, Downing Street has said.
Mr Blair's work schedule is not expected to be disrupted
Mr Blair was given an anti-inflammatory injection for back pain at London's Royal Free Hospital, said Number 10.
He had been suffering from discomfort for some months and thought he had injured his back in the gym.
No 10 says Mr Blair is now at his Chequers country residence and was not expected to need further treatment, nor would it affect his job in any way.
His GP referred him to the hospital for the treatment and Mr Blair had the treatment at about 1900 BST on Thursday.
BLAIR HEALTH PROBLEMS
October 2003: Admitted to hospital with irregular heartbeat
November 2003: Seen by doctors twice in 24 hours after complaining of stomach pain
October 2004: Treated again for irregular heartbeat
Mr Blair's wife Cherie denied Mr Blair had a slipped disc when asked about his health at a law awards meeting.
But Downing Street explained Mrs Blair had been told her husband had a "prolapsed disc" and did not realise it was the same as a slipped disc.
No 10 says the back pain will not affect his plan to serve a full term in office.
The prime minister held talks with European Commission President Jose Manuel Barroso as planned on Friday, but the location of the meeting was changed from Downing Street to Chequers.
Downing Street is playing down the treatment but it follows his treatment for an irregular heartbeat last October.
BBC chief political correspondent Mark Mardell said there had already been speculation about how long Mr Blair would want to continue as prime minister if he had health problems.
Consultant radiologist Andrew Platts, who treated Mr Blair, suggested he was unlikely to need further treatment after the injection.
"For the majority of people, this treatment is sufficient," said Dr Platts.
In a statement, Downing Street said: "The prime minister has been experiencing some back pain over the last couple of months.
"It has given him discomfort from time to time, but obviously hasn't stopped him doing his job.
"Following a referral by his GP, he had an outpatient appointment at the Royal Free Hospital earlier this evening. Following treatment he went to Chequers."
Some journalists had noticed Mr Blair appeared to be walking oddly during the election campaign - one member of the public even asked him about it.
Downing Street was unable to say whether this was linked to his back pain.
Mr Blair's health has hit the headlines three times before.
He was treated for an irregular heartbeat in October 2003 and October 2004.
And in November 2003 he was seen by doctors twice in 24 hours after complaining of stomach pain but given the all clear.
Mr Blair is known for keeping fit through playing tennis and running on the treadmill.