Britain's economy picked up speed in the third quarter thanks to a strong services sector, official data confirmed Friday, ahead of years of expected weaker growth on gathering Brexit clouds.

Gross domestic product grew 0.4 percent in the third quarter, up slightly from 0.3 percent in the April-June period, said the Office for National Statistics (ONS) as it reiterated last month's second estimate.

"Today's unrevised third-quarter figures show most of the growth came from the dominant service sector, with accounting, recruitment agencies and retailing all performing well," said ONS Head of National Accounts Darren Morgan.

"Manufacturing also boosted growth thanks to an increase in exports and the introduction of new car models. Meanwhile, household spending and business investment both grew steadily."

The government had however warned last month in its annual budget that the economy would grow much slower than expected over the next five years.

Britain remains on course to leave the European Union in March 2019 following last year's shock Brexit referendum.