Green Party TD Paul Gogarty accused Fine Gael of "missing the point", while Labour's Transport spokeswoman, Roisin Shortall, expressed "doubts" about the model proposed. The row comes as an Irish Times opinion poll shows that the potential rainbow coalition has fallen behind the government in terms of public support.

Fine Gael's transport spokeswoman Olivia Mitchell launched her party's policy document on Friday, advocating the introduction of competitive tendering of existing and new routes and the establishment of a bus regulator.

Mitchell said the introduction of this London-style, "limited competition" model "would radically transform the bus market in Dublin".

But the proposal would be fiercely resisted by the public sector unions and Labour's Roisin Shortall said that it was "not the model I would be suggesting". While she agreed with Mitchell's emphasis on the pivotal role of buses, Shortall questioned whether valid comparisons could be made between Dublin and London.

"London was a de-regulated, chaotic model before 'Transport for London' was set up.

Here we have a coherent service, although it is inadequate and underfunded."

Suggesting the establishment of a regulator's office could cost 35m per year, Shortall said she did not know "where Fine Gael was getting its costs from" when it said there would be no significant costs. Labour does not rule out the use of private sector buses, but it favours Dublin Bus . . . not a regulator . . . managing and planning the network.

The Greens' Paul Gogarty issued a statement saying real investment in an affordable service was needed to get people into buses. "Fine Gael needs to be honest and state that there will be a cost implication, which will pay for itself in the long run."

However, Shortall played down suggestions that the differences did not augur well for a rainbow government. "That's what coalition is about. We [FG and Labour] are two different parties with distinct policies. If we are negotiating a government, dealings will be done at that stage and we will hammer it out then."