WA Labor will not field a candidate in the Cottesloe byelection prompted by Colin Barnett's resignation.

In a statement released on Wednesday morning, state secretary Patrick Gorman conceded the party could not win the safe Liberal seat.

Premier of Western Australia, Colin Barnett and wife Lyn cast their votes in the last state election. Credit:Tony McDonoough

"WA Labor will not contest the Cottesloe byelection caused by the impending resignation of Colin Barnett," he said.

"While I know we would achieve a strong swing to WA Labor and our members and campaigners would do us proud, the truth is we would not win the seat.