A SINN Féin councillor who resigned from the party this weekend has launched an unprecedented attack on his former colleagues and the party leadership, and accused members of "sycophantism".

Dublin city councillor Killian Forde, who left the party over a row with his party colleagues on the council, said: "Sinn Féin over-value loyalty and obedience when there is a need to embrace criticism, debate and discussion."

The resignation of Forde, who is expected to join Labour, is the latest blow to a party rife with divisions over its political direction and questions over the leadership.

Speaking exclusively to the Sunday Tribune in the days before his resignation, Forde said: "Sycophantism in any organisation is dangerous – in a political party it is cancerous.

"There are obviously problems with succession in Sinn Féin, particularly as they need a public face in the south. Assumptions have led to the problem that Mary Lou [McDonald] finds herself in a constituency where it is very difficult to see how she can possibly win a seat."

McDonald lost her seat as an MEP for Dublin last June and she failed to win a Dáil seat in Dublin Central in 2007. Although she is well-respected within Sinn Féin, Forde's lack of confidence in McDonald's ability to win a seat is not uncommon within the party and he also questioned the political direction of the party in the south.

"Sinn Féin drifted into the centre following a decent 2004 election," he said. "Flirting with Fianna Fáil and assuming they would be junior coalition partners was a massive error, one that was driven by the arrogance of having a dozen successive election successes and not welcoming alternative viewpoints."

Forde is the third Sinn Féin councillor to resign from Dublin City Council, cutting the party's representation from seven to four councillors since last June's elections.

Forde's brazen attack on his own party stems from his decision, as chairman of the finance committee on Dublin City Council, to vote in favour of the council's 2010 budget on 21 December while his four party colleagues on the council voted against it.

An initial draft budget developed by council officials was deemed too harsh on vulnerable people by Labour and Sinn Féin councillors.

So over a nine-week period councillors worked to produce a more equitable budget and this was voted on at the 21 December meeting. The other four Sinn Féin councillors voted against the budget because of a proposal to amend the waiver for the lifting of black refuse bins, claimed Forde.

"In my view it was pointless to take a stand on this issue, as we elected councillors who have no role in setting or removing waste charges. The power to do so lies with the city manager.

"For me to have spent nine weeks working in good faith trying to get the budget from 'utterly grim' to 'okay' and then not even vote for it would have been opportunistic and cynical. And so against the instructions of the party I proposed and voted for the budget."

Forde told party officials he was resigning from the party on Friday. Sinn Féin has called on him to resign his seat to allow a co-option of a new Sinn Féin councillor.