FLC OF AN LTT ACR ACFT INADVERTENTLY ENTERED A PROHIBITED AREA AS A RESULT OF ATTEMPTING TO ADJUST THEIR SPACING BEHIND ANOTHER ACFT ON A VISUAL APCH.

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ACR FLT FROM RIC-DCA WAS CLRED FOR THE VISUAL RWY 15 AT DCA. TWR ASKED IF WE HAD 727 TFC IN SIGHT THAT WAS DOING RWY 18 RIVER VISUAL. TFC WAS ACKNOWLEDGED. TWR ASKED IF ARPT WAS IN SIGHT. ARPT WAS ACKNOWLEDGED. RWY 15 WAS ALSO IN SIGHT AND WE WERE CLRED FOR THE VISUAL APCH TO RWY 15. APPROX 3-4 MI FROM RWY 15; TWR THEN CLRED US FOR THE RIVER VISUAL TO RWY 18. MY CAPT ASKED TO CONFIRM THIS AND IT WAS CONFIRMED BY TWR. WE NOW WERE WITHIN 2 MI OF THE 727 TFC WE WERE TO FOLLOW. WE SLOWED OUR ACFT AND BROKE OFF THE APCH TO RWY 15 AND SQUARED OFF THE R BASE FOR RWY 18 TO ALLOW SEPARATION WITH THE 727 TFC. WE WERE KEEPING WATCH OVER THE RIVER NOT TO STRAY INTO THE P-56 AREA. TWR ADVISED US WE WERE STRAYING INTO P-56 AND ASKED US TO MAKE A HARD L TURN AND WE WERE GIVEN VECTORS FOR THE RWY 21 VISUAL. THE HARD L TURN TWR ADVISED SEEMED TO PUT US INTO THE P-56. WE CONTINUED FLYING TWR ASSIGNED VECTORS AND LANDED ON RWY 21 WITHOUT ANY FURTHER INCIDENT. AT NO TIME DID WE APPEAR TO PLACE OTHER ACFT IN DANGER. THE WX WAS 2500 SCATTERED; 6500 BROKEN WITH 10 MI VISIBILITY AND LIGHT RAIN SHOWERS BUT AT OUR ALT IT WAS LESS.