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Oct 11 (Reuters) -The blue-chip FTSE 100 was flat on Friday, weighed down by losses in Sainsbury's after its biggest shareholder cut stake in the supermarket chain, while BP slipped as it warned on third-quarter profit.

Sainsbury's SBRY.L dropped 4.2%, the top decliner on the FTSE 100, after the Qatar Investment Authority looked to sell 306 million pounds ($399 million) worth of shares.

British oil major BP BP.L slipped 0.4% after it said weak refining margins would dent its third-quarter profit by up to $600 million.

The FTSE 100 .FTSE was little changed at 0752 GMT, on track for a second consecutive week of declines.

Stocks took little comfort from data that showed Britain's economy grew in August after two consecutive months of no growth.

Economic output rose by 0.2% in monthly terms in August, in-line with economists' expectations, and was likely to reassure Finance Minister Rachel Reeves ahead of the new Labour government's first budget.

"The bottom line is that the economy still seems to be growing at a reasonable pace, but the 0.6/0.7% quarterly GDP readings we became accustomed to in the first two quarters of the year are not going to be repeated in the second half of the year," James Smith, economist at ING, said.

The midcap FTSE 250 index .FTMC edged up 0.1%, but was also headed for a weekly decline.

Jupiter Fund Management JUP.L slipped 1.2% after it reported lower assets under management with outflows of 1.6 billion pounds in the third quarter.

Saga SAGA.L jumped 9.6% after it said Belgian insurer Ageas AGES.BR is in exclusive talks to set up a 20-year motor and home insurance broking partnership with the British over-50s holiday group.

Reporting by Sruthi Shankar; Editing by Mrigank Dhaniwala

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