ISTANBUL (REUTERS) - A Turkish Airlines flight to Lisbon was diverted back to Istanbul on Wednesday after unaccompanied baggage was found on board, a spokesman told Reuters.

TK1759 was carrying 170 passengers and seven crew members to Lisbon, the spokesman said on condition her name was not used.

All passengers were being taken to another aircraft to resume their journey to Lisbon, she said.

It was the third time in a week a Turkish Airlines flight has been forced to change course. A Sao Paulo-bound flight landed in Morocco after a note with the word "bomb" was found in the lavatory on Monday.

On Sunday, a flight from Istanbul to Tokyo turned back shortly after take-off when another note was found on the lavatory door.

Turkish Airlines' website said TK1759 had been scheduled to depart at 11.45am (0845 GMT). It was not immediately clear how far the plane had travelled before turning back.

The Portuguese airport authority said the flight had not reached its airspace.

Turkish Airlines, Europe's fourth-biggest carrier, carried almost 55 million people last year.