LONDON (Reuters) - British economic growth slowed in the three months to August and there is a significant chance of a recession by the end of next year, the National Institute of Economic and Social Research, a leading think tank, said.

NIESR estimated the economy expanded by 0.3 percent in the three months to August, slowing from growth of 0.4 percent in the three months to July, chiming with signs the impact from June's Brexit vote is likely to be gradual rather than instant.

Previously released official figures have shown the economy expanded by 0.6 percent in the second quarter, boosted by household spending.

NIESR said the economy has been broadly flat since April, leaving a "significantly elevated" chance of a recession before the end of 2017.

"The evidence on the current state of the economy post-referendum is limited, but on balance these data suggest that the UK economy is in the midst of a slowdown," NIESR research fellow Rebecca Piggott said.

((Reporting by Andy Bruce))