Turkey's parliament has passed a bill revamping electoral regulations, a move the opposition says could open the door to fraud and jeopardise the fairness of 2019 elections.

The legislation formally allows the creation of electoral alliances, paving the way for a tie-up between President Recep Tayyip Erdogan's ruling Justice and Development (AK) Party and the nationalist opposition.

The AK party and opposition Nationalist Movement Party (MHP) submitted the bill to the parliament speaker's office last month.

The bill, which comprises 26 articles, grants the Supreme Electoral Council the authority to merge electoral districts and move ballot boxes to other districts.

Ballots will be admissible without the stamp of the local electoral board, formalising a decision made during a referendum last year that caused a widespread outcry among government critics and concern from election monitors.

Under the bill, security force members will be allowed into polling stations when invited by a voter, a measure the government says will stamp out intimidation by the outlawed Kurdistan Workers Party (PKK) in the mainly Kurdish southeast.

The bill also states a political party can officially back one another during elections, adding that votes received by the alliance will be counted separately for each party.

The number of elected MPs representing the alliance will be based on the total number of votes it gets.

Furthermore, the MPs representing each party of the alliance will be decided based on the specific number of votes it gets.

The alliance parties may together extend the 10 percent election threshold, a requirement for representation in the 550-seat parliament.

The parties that decide on making an alliance should submit an application to the Supreme Election Council within seven days after the election calendar is announced.

MHP backs AK party

On January 8, MHP leader Bahceli announced that his party would support AK party leader and President Recep Tayyip Erdogan in the 2019 presidential elections, rather than field a candidate of its own.

Ahead of a constitutional referendum last April, both the AK party and MHP campaigned for the sweeping package of amendments that changed Turkey's parliamentary system to an executive presidency a month later.

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The overhaul comes about 18 months ahead of the upcoming pivotal elections in Turkey.

Next November, the country is scheduled to pick a new parliament and formally concentrate executive power in the office of the president in line with the changes made in the 2017 referendum.

The changes are a "serious threat" to fair and free polls, the main opposition Republican People's Party (CHP) said in a statement on Thursday.

The CHP criticised the presence of security forces at polling stations, which MPs said could be used to make vote counting less transparent.

The pro-Kurdish Peoples' Democratic Party (HDP) has said the measures could lead to ballot boxes being moved out of districts where it has strong support.