Chancellor George Osborne has said the UK continues to outperform other major economies as figures showed the economy grew by 0.5% between July and September.
The figure is down from 0.7% in the second quarter.
GDP is 0.5%. UK continues to outperform other major economies. But global risks mean we go on with tough decisions to live within our means
The UK economy grew by 0.5% in the third quarter of 2015, down from 0.7% in the second, the Office for National Statistics said.
In the week that the Conservatives focused their campaign on the economy, the Office for National Statistics released disappointing growth figures.
The UK grew by 0.3% during the first quarter of 2015, half the 0.6% rate seen in the final quarter of 2014 and the weakest quarterly growth since the end of 2012.
David Cameron said it was a "timely reminder that we cannot take recovery for granted".
ITV News' James Mates reports:
Nicola Sturgeon has said that today's slow economic growth figures " highlight the need for an alternative to austerity".
Ms Sturgeon called for "modest spending increases" and "investment in actions to get our economy growing faster, to get more people into jobs and to raise our revenues".
UKIP leader Nigel Farage has said that today's slow economic growth figures "will be very disappointing for the government".
He said: "Their whole general election pitch is that there's growth in the economy... I've felt for too long that actually not many people are feeling the benefits."
Liberal Democrats Treasury Secretary Danny Alexander insisted the economy was continuing to make progress but that more needed to be done.
"The British economy is recovering well, but these figures remind us that there is still work to do to secure the recovery," he said.
Responding to today's disappointing economic figures, the shadow Chancellor Ed Balls said that the Conservatives "have not fixed the economy for working families".
He said: “Tory economic policy may be helping a few at the top but for most people bills have gone up faster than wages, which are down £1600 a year since 2010.
"The Tories just don’t understand that Britain only succeeds when working people succeed."
The Chancellor George Osborne has insisted that it's good news that the economy is growing, even though the latest figures are half as strong as at the end of 2014.
ITV News Economics Editor Richard Edgar spoke to George Osborne this morning:
Britain's economy is still growing, but it grew much slower in the first quarter of 2015 than it did at the end of last year.
Official figures show that GDP grew by 0.3% between January and March this year - half as much as it grew between October and December 2014, at 0.6%.
This is the weakest quarterly growth since the end of 2012 - and with the General election just nine days away, this will be seen as a major setback for the government.
The ONS puts the blame on a slower service sector, a shrinking construction industry, and a slight squeeze on industrial production caused by lower oil and gas prices.
ONS chief economist Joe Grice said: "As always, we warn against reading too much into one quarter's figures."