The forint weakened after the Hungarian central bank unexpectedly lowered the interest rate on its foreign-currency swaps following one of the biggest emerging-market currency rallies this year.

The move, which takes the implied rate offered on foreign-currency swaps down to 5.25% from 5.75%, is testing investors as the National Bank of Hungary looks for ways to limit the gains of the forint, which appreciated as much as 10% since lows hit in March.