Red Bull's team management is under pressure from the Milton-Keynes design office, headed by Adrian Newey, to confirm the brand of engines for the 2010 season.
The 2009 title contenders are openly musing their options for next year's campaign, with Mercedes-Benz tipped as the leading alternative to Red Bull's current supply of Renault's 2.4 litre V8s.
But McLaren has the right to veto works partner Mercedes' arrangements with other teams, which is believed to be the reason for the delay.
Germany's Auto Motor und Sport reports that staying with Renault power, or even switching to Cosworth, are options for Red Bull Racing.
Amid the current engine freeze, 2010 returnee Cosworth is currently in the phase of retuning its 2006-spec V8 unit for the new regulations.
It also emerges that the British company has appointed four engineers from Mercedes-Benz for its new F1 programme, and Auto Motor und Sport said it is rumoured that Cosworth might be on the verge of an alliance with Porsche.
