The Organization of Petroleum-Exporting Countries (OPEC) is no longer a "viable entity" following its refusal to cut oil production in November, Dennis Gartman, founder and editor of the closely-watched The Gartman Letter, told CNBC on Tuesday. "I really do think that we have seen the end of OPEC as a viable entity," Gartman told CNBC Europe's "Squawk Box" on Tuesday. "I think it's still broken. We'll see what happens in a year from now but I honestly think that OPEC, as it has been in the past, is a finished issue." Gartman's comments come amid tumbling oil prices , with Brent crude falling below $66 a barrel on Tuesday - its lowest level in five years. The price has fallen over 40 percent since June on the back of a global over-supply and lack of demand.
In November, the group of 12 major oil producers, OPEC, decided not to cut production at its meeting in November, further fueling sliding prices.
Read More Oil falls as OPEC opts not to cut production Some OPEC members, such as Iran, Venezuela and Ecuador, indicated that they were keen to cut production in order the stem the fall in prices. But OPEC's largest oil exporter, Saudi Arabia, refused to budge. Gartman said the leadership position of Saudi Arabia - the main swing producer in the region - had "strengthened". "(Now) they (OPEC) are doing exactly what the Saudis want to be done, as the Saudis continue to defend their market share so that they can put pressure on their enemies the Iranians, their other enemies of ancient times, the Russians and their new enemies, the frackers (U.S. shale oil producers)," he said.
Read More OPEC needs to 'wake up' to shale revolution "But are the other members of OPEC happy about what's happening? Is Venezuela or Iran happy with what's going on? Of course not. I think we've seen the end of OPEC as a viable entity." It's not the first time that Gartman has criticized the group of oil exporters. Following OPEC's November meeting, Gartman told CNBC: "The fact that OPEC, for all intents and purposes, is broken apart and is really now a broken cartel only serves to make certain that crude oil prices are still going to fall even further". Global markets fell overnight as oil prices continued to decline in the Asian trading session. The decline in oil hit U.S. stocks hard on Monday, with the Dow Jones Industrial Index posting its biggest decline since October.
Read More Dow's biggest dip since October as stocks hit by oil Aside from the market jitters caused by falling oil prices, investors are also worried about the U.S. Federal Reserve's next move - and when the central bank will raise interest rates.
Given this uncertainty, Gartman added that it was a difficult time to enter the stock market and advised investors to "go to the sidelines". "The bulls are taking money off the table because they want to protect what they have, the bears are finally saying: 'You know what, I'm taking my money off the table because I'm not going to punt and try to make my money back'. And illiquidity seems to reign," the investor said.
He added that illiquidity begets more illiquidity, and that investors should go to the sidelines and become "a little bit smaller, make your positions smaller - but to try to play catch up is probably ill-advised". - By CNBC's Holly Ellyatt, follow her on Twitter @HollyEllyatt. Follow us on Twitter: @CNBCWorld