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.”
Weary London commuters were tonight offered little in the way of hope as another 48-hour tube strike appeared inevitable.
A new war of words erupted today in the increasingly bitter dispute over ticket office closures on London Underground.
A fresh attempt to halt next week's tube strike was thrown into doubt as it emerged the RMT union may boycott new peace talks.