The lira hit a record low of 26.9 against the U.S. currency, sliding from Monday's close of 26.3505.
The central bank will announce its rate decision at 1100 GMT on Thursday.
"News saying that the central bank's interest rate hike will be below market expectations is triggering the lira depreciation," said one trader.
But June's hike was below expectations, with economists saying Erdogan's influence over the central bank limits how far they can go in tightening policy.
Turkey's annual inflation surged to a 24-year high of 85.51% last October, mainly due to lira depreciation because of Erdogan's policy of low rates.
Persons:
Abdulkadir Selvi, Tayyip Erdogan, Nevzat Devranoglu, Daren Butler, Christina Fincher
Organizations:
Hurriyet, Ece Toksabay, Thomson
Locations:
ISTANBUL