* Dalian iron ore drops over 8% to lowest since June 8

* SGX iron ore falls over 5% to weakest since June 8

* Dalian coking coal, coke snap 7-session rally

* China launches probe into iron ore spot trading

June 21 (Reuters) - Iron ore futures in Asia tumbled on Monday, with losses widening after the Chinese authorities launched an investigation into the spot market as prices of the key steelmaking ingredient remained high despite repeated warnings against hoarding and speculation.

The most-traded September iron ore on China's Dalian Commodity Exchange ended daytime trading 8.8% lower at 1,121 yuan ($173.31) a tonne, after earlier hitting 1,118.50 yuan, its weakest since June 8.

The most-active July contract on the Singapore Exchange fell as much as 5.7% to $195.05 a tonne, the lowest also since June 8.

China's state planner, the National Development and Reform Commission (NDRC), and the State Administration for Market Regulation vowed to "strictly punish and disclose" market irregularities such as hyping prices and hoarding.

The authorities visited the Beijing Iron Ore Trading Center to discuss works to ensure supplies and stabilise prices, according to a NDRC statement.

Spot prices had held ground above $200 a tonne over the past three weeks, despite government efforts to rein in commodity inflation partly driven by speculative trades that helped propel iron ore to a record peak above $230 last month.

Both Dalian and SGX benchmarks, however, were already under pressure when trading began, as sentiment was hit by a seasonal slowdown in construction activity in top steel producer and consumer China.

"It's safe to say that China's seasonal slowdown in construction steel demand has now arrived, with rebar inventories at steel mills and warehouses across China both showing firm builds," said Atilla Widnell, managing director at Navigate Commodities in Singapore.

Steel futures also fell, with rebar on the Shanghai Futures Exchange down 4.2%. Hot rolled coil fell 4.3%, while stainless steel slipped 0.6%.

Dalian coking coal slid 4.2% and coke shed 5.4%, after seven straight sessions of gains, as the coal market has also come under similar government scrutiny.

(Reporting by Enrico Dela Cruz in Manila, Editing by Sherry Jacob-Phillips and Shounak Dasgupta)