* BP says third-party estimates assume wrong pipe diameter

* Says data indicates about 50 pct of plume is gas

* BP says providing U.S. government with all information

* Shares in BP down more than 3 pct on cover-up allegations

LONDON, May 21 (Reuters) - BP Plc BP.L said some third-party estimates of how much oil is gushing out of a leaking well on the sea floor of the Gulf of Mexico were inaccurate and it denied a cover-up.

BP has been estimating that the leak was flowing at a rate of 5,000 barrels a day, but some scientists and the U.S. government have questioned that figure. [ID:nN19272725]

“It was only ever meant to be a rough estimate,” a spokesman for the British company said on Friday after U.S. lawmakers accused it of concealing the extent of the spill and as TV images showed oil sloshing into Louisiana’s marshes. [ID:nN20262611].

BP also issued a statement saying a number of methods and variables could be used in calculating the flow. Damage to the end of the pipe had distorted its diameter by about 30 percent, while a drill pipe trapped inside the riser had reduced the flow area by an additional 10 percent, it noted. [ID:nRSU3465Ma]

"Thus, some third party estimates of flow, which assume a 19.5 inch diameter, are inaccurate," BP BP.N said.

The company also pointed out that data indicated about 50 percent of the plume formed by the leak was natural gas rather than oil.

BP said it was in the process of providing the U.S. government’s Flow Rate Technical Team with all requested information.

Delays in getting live a video link of the plume onto the Internet were due to “some simple technical difficulties”, the BP spokesman said.

Shares in BP were down 3.4 percent at 511 pence by 1202 GMT, having earlier struck a nine-month low on the back of the cover-up allegations. [ID:nLDE64K0KJ] (Reporting by Tom Bergin, writing by Paul Hoskins, editing by Will Waterman)