CARACAS, Feb 25 (Reuters) - Venezuela's black market exchange rate weakened below 200 bolivars per dollar on Tuesday, according to a widely referenced website DolarToday, despite the recent launch of a foreign exchange platform meant to reduce pressure on the black market.

The bolivar has weakened 14 percent since the start of the year and 56 percent in the last 12 months, according to DolarToday, which publishes the black market rate based mostly on currency trades along the Colombian border.

The government last week opened a "free-floating" currency exchange mechanism known as Simadi which currently sells dollars for 172 bolivars. That is the weakest of a three-tiered exchange control system that also sells dollars at a preferential rate of 6.3 bolivars for imports of food and medicine and a complementary rate of around 12 bolivars for other goods.

The launch of Simadi was meant to slow the depreciation of the bolivar on the black market and improve supplies of hard currency in order to limit nagging shortages of staple goods.

Venezuelans attempting to use Simadi, particularly those involved in small-scale cash transactions, have complained of delays in obtaining greenbacks.

"On Friday I was very confident about the system, I (deposited) 34,000 bolivars," said Jorge More, 37, who works in information technology, in an interview on Tuesday. "But I haven't been authorized to receive the dollars. I'm not happy with the lack of clarity."

Officials have said state oil company PDVSA will sell dollars through Simadi to ensure sufficient supply of currency, but finance industry sources say the government has not yet approved the regulations needed to make this possible.

(Reporting by Girish Gupta and Alexandra Ulmer, writing by Brian Ellsworth; Editing by Tom Brown)