David Cameron has labelled next week's planned Tube strike as "completely unjustified and unacceptable".
The RMT union has called five days of strike action amid an ongoing row over Tube ticket office closures.
Talks between London Undergorund and the RMT have so far failed to find a resolution and avert the strike.
“Next week’s Tube strike is completely unjustified and unacceptable," the Prime Minister told the Evening Standard.
“It will hit millions of hard-working families across the capital and cause chaos for businesses. Ed Miliband must make clear he condemns this strike without reservation.”