* FTSE 100 down 0.4%, FTSE 250 off 0.1%

Sept 4 (Reuters) - The FTSE 100 fell on Friday and was set for its third straight weekly loss following a tech-fuelled plunge on Wall Street, while Barratt and Persimmon tumbled as the UK's competition regulator began an enforcement action on certain housebuilders.

The Competition and Markets Authority (CMA) said it was opening enforcement cases to focus on certain practices of Barratt Developments, Persimmon Homes, Taylor Wimpey and Countryside Properties, saying they might have broken a consumer protection law in relation to leasehold homes.

Shares of all four housebuilders fell between 1% and 3.5% in early trading, with the wider housebuilding index shedding 1.2% to its lowest in more than two months.

The blue-chip FTSE 100 lost 0.4%, while the mid-cap index FTSE 250 dipped 0.1%. (Reporting by Shashank Nayar in Bengaluru; Editing by Subhranshu Sahu)