Political parties on Monday failed to iron out differences over a long-delayed electoral system bill aimed at introducing reforms before April¡¯s general elections.

In a move to bring the stalled bill to the National Assembly¡¯s extraordinary session, three opposition parties called for Speaker Park Kwan-yong to convene a committee of the whole house. But the pro-government Uri Party blocked the attempt and the opposition backed down.

A rupture in negotiations on the most contentious issue _ the number of lawmakers to be elected under proportional representation _ has clouded prospects of passing the controversial reform bill within this year.

Under a recent ruling by the Constitutional Court, the current electoral district system and the 272-member parliament will be left unconstitutional unless a special committee for political reform legislates a new system by the year¡¯s end.

Rep. Mok Yo-sang, chairman of the special committee, said he will step down on Tuesday to take responsibility for the ongoing dispute over the electoral system.

For the past couple of weeks, the Assembly¡¯s special committee has been crippled as the Uri Party and three opposition forces refused to give ground over the issue.

Lawmakers of the majority Grand National Party (GNP), the Millennium Democratic Party and the United Liberal Democrats used their power in the special committee to push for a vote on their proposals. But Uri Party officials barred their attempts by taking up all the seats in the committee room and blocking them from discussions.

The GNP and other opposition parties have agreed to keep the number of lawmakers at 273, but they want just 30 selected under the proportional representation system.

The Uri Party wants with 46 seats filled under proportional representation. Uri Party lawmakers argue that the opposition parties are seeking to increase the number of regional representatives due mainly to their vested interests.

The opposition parties had urged the National Assembly speaker to forward their proposals at the plenary session, acting ex officio.