The UK economy grew by 0.1% between April and June, less than the 0.2% estimated previously.
Output from the service sector grew by 0.2% in the quarter, compared with the previous estimate of 0.5%, the Office for National Statistics (ONS) said.
Separate data suggested activity in the UK service sector grew in September.
The Treasury pointed to these figures as evidence the UK economy was still growing and said it would be sticking with its deficit reduction programme.
The ONS also revised down growth in the first three months of the year, from 0.5% to 0.4%.
It added that household consumption fell by 0.8% in the second quarter.
Separately, a survey from Markit/CIPS found the UK's service sector purchasing managers' index (PMI) recorded a figure of 52.9 for September, up from 51.1 in August. A figure above 50 indicates growth.
Figures from the same company published earlier this week showed surprise growth in the manufacturing sector.
A Treasury spokesperson said: "While the UK cannot insulate itself from what is happening to our major trading partners, with financial turbulence in the eurozone and a weaker outlook for global growth, the economy is still growing and this week's survey data for the manufacturing and service sectors are consistent with continued expansion."
The government has been criticised in some quarters for concentrating too much on cutting the budget deficit at the expense of stimulating growth.
On Wednesday, the International Monetary Fund (IMF) said that Europe's stronger economies should avoid imposing drastic budget cuts at the expense of growth.
If economic conditions deteriorate in the UK, Germany or France, governments should "consider delaying" cuts, particularly as they can borrow at low interest rates, the IMF said.
However, the Treasury reiterated that it did not intend to hold back on its spending cuts.
"The government will stick to the deficit reduction plan which has won the UK credibility and stability, but the most important thing for the economy now is restoring confidence, which will depend on the eurozone decisively dealing with its problems."
The latest GDP revision is likely to raise further questions about the strength of the UK's fragile economic recovery.
In an interview with the Reuters news agency, the head of the British Retail Consortium, Stephen Robertson, said: "The next six months are going to be characterised by very low levels of growth. I think we've probably got another 18 months of real challenge."
Underlying costs for businesses will continue to rise, with the retail sector finding life "extremely difficult", he added.
On Wednesday, supermarket giant Tesco reported a fall in like-for-like sales excluding petrol and VAT of 0.5% for the first half of the year, while retailer Mothercare said like-for-like sales between July and September fell by almost 10%, triggering a 36% slump in its share price.
Also on Wednesday, airline Flybe saw a similar fall in its share price after reporting a "significant slowdown in sales" across its UK domestic network in September.
There has been speculation that the Bank of England will restart its quantitative easing (QE) programme, whereby it pumps money into the economy to boost demand.
At its meeting last month, most members of the Bank's Monetary Policy Committee (MPC) agreed the case for an "immediate" stimulus had strengthened.
Some analysts said the encouraging PMI data could mean a delay in restarting QE, but most agreed further stimulus would be introduced this month or next.
The MPC's next meeting begins on Wednesday, with an announcement on interest rates and QE due at midday on Thursday.
"The bigger picture, including the risks to the economy from the eurozone debt crisis, far outweigh the services PMI survey and our view remains that the MPC will probably sanction further asset purchases tomorrow," said Philip Shaw at Investec.
Brian Hilliard at Societe Generale said the continuing debt crisis "should trigger a move [for QE] this week. I give about a 60% chance to that; if not, it's an absolute racing certainty for November".