There is mounting speculation that Prime Minister Tony Abbott will face a challenge for the leadership from one of his closest allies.

While some senior ministers have leapt to the defence of their embattled leader others have started to circulate letters mustering support for a leadership spill.

Two of his most ardent supports have emerged as the likely challengers.

An exclusive Fairfax Media report revealing Communications Minister Malcolm Turnbull and Foreign Minister Julie Bishop are considering their leadership options for the first time.