Pakistan's Supreme Court on Tuesday struck down Punjab Assembly deputy speaker's controversial decision to reject 10 votes during the chief minister's election as unconstitutional and named PML-Q leader Chaudhry Parvez Elahi as the new executive head of the politically crucial province, in a major setback to Prime Minister Shehbaz Sharif and the ruling coalition.

Elahi, who lost the election held on Friday despite getting a majority vote, challenged the ruling of deputy speaker Sardar Dost Muhammad Mazari who handed victory to Hamza, son of Prime Minister Sharif. Elahi was supported by ousted Prime Minister Imran Khan's Pakistan Tehreek-e-Insaf party.

A three-member bench comprising Chief Justice Umar Ata Bandial, Justices Ijazul Ahsan and Munib Akhtar, which heard the case, ruled that 76-year-old Elahi is the chief minister of Punjab province.

The court also ordered the Punjab Governor to swear in Elahi as chief minister before 11:30 PM Tuesday night. In case the governor does not administer an oath to Elahi, President Dr Arif Alvi can do so instead, the court ruled in its short order.

The court also declared all the appointments made by Hamza illegal and told the members of his cabinet to vacate their offices.

Earlier, Chief Justice Bandial had said that the judgement would be issued at 5:45pm and later it was scheduled to be announced at 7:30pm. However, the verdict was announced after a three-hour delay via short order.

Elahi secured 186 votes against 179 votes of Pakistan Muslim League-Nawaz (PML-N) candidate Hamza Shehbaz but lost after Deputy Speaker Mazari rejected 10 votes of his Pakistan Muslim League-Quaid (PML-Q) lawmakers.

Mazari refused to count the vote after PML-Q chief Chaudhry Shujaat Hussain through a letter directed his party lawmakers to vote for Hamza, son of Prime Minister Sharif and PML-N president.

The Punjab province has been in turmoil since April which took a new turn on Friday when Pakistan Tehreek-e-Insaf supported candidate, Elahi, was not declared a winner.

Mazari's ruling was challenged by Elahi and the court on Saturday asked Hamza to continue as trustee chief minister.

The apex court order irked the coalition government led by which after criticising it demanded the constitution of a full bench, and reiterated it on Monday as the court grappled with the issue.

On Monday, the Supreme Court refused to form a full bench to hear the case related to the re-election of Hamza, saying it will have to hear more arguments before taking a decision on the issue.

Jamiat Ulema-I-Islam Fazl (JUI-F) chief Maulana Fazlur Rehman at a press conference in Islamabad said that the government lawyers recommended a full bench but the court instead of reflecting and accepting the advice, rejected it.

JUI-F is a coalition partner in the Pakistan government headed by Prime Minister Sharif's Pakistan Muslim League-Nawaz (PML-N).

Pakistan's coalition government criticised the apex court's decision not to form a full bench and boycotted the court proceedings on Tuesday.

Foreign Minister Bilawal Bhutto-Zardari, who is Pakistan People's Party chairman, had reaffirmed the boycott call, saying the demand for a full court was for the sake of the Constitution, democracy and the court's own integrity.

PML-N Vice President Shahid Khaqan Abbasi had said it was a "test" for the apex court since it was a requirement of justice that a judge or bench recuse themselves from a case in which fingers were raised on them.

The security was tightened around the court building where a large number of police officials were deployed. Paramilitary Rangers, and Frontier Corps personnel were also present to help the police.

Punjab is Pakistan's second largest province and any party ruling the key province controls the politics of the country.

For the PML-N, losing Punjab means losing the Centre. Political pundits believe that the ruling coalition is now vulnerable. Tuesday's outcome provides a "massive opportunity" to the PTI chairman to build further pressure on the rulers to call fresh elections and also corner the PML-N in Punjab which is considered a stronghold of Sharifs.