Britain's Labour Party Voting starts between Jeremy Corbyn and Owen Smith

This file photo shows UK Labour's current leader Jeremy Corbyn (L) and leadership challenger Owen Smith (R) during a debate.

Britain's Labour Party has started voting to decide if Jeremy Corbyn will remain the leader of the country’s main opposition party or if Owen Smith should take over.

Voting will begin on Monday to decide the fate of the Labour Party’s next leader, who is expected to run for Prime Minister in the next general election in 2020.

Despite strong support for Corbyn among party members, he has suffered a series of major blows as senior Labour lawmakers have endorsed Smith instead.

The Scottish Labour leader, Kezia Dugdale, declared her support for Smith over the 67-year-old veteran socialist on Monday.

Dugdale, who led Labour to its worst defeat in the Scottish elections earlier this year, wrote in the British paper Daily Record: “I don’t think Jeremy [Corbyn] can unite our party and lead us into government. He cannot appeal to a broad enough section of voters to win an election,” she wrote. “I believe Owen can.”

This comes just as Labour Mayor of London, Sadiq Khan, called on his party’s members to ditch Corbyn over the weekend and asserting that only Smith can lead the party forward.

Khan criticized Corbyn for being unfit for leadership, saying, “Jeremy has already proved that he is unable to organize an effective team and has failed to win the trust and respect of the British people.”

London Mayor Sadiq Khan backs Owen Smith as next Labour leader (AFP).

The Labour Party is engulfed in a bitter internal power struggle between Corbyn's supporters in the grassroots membership and the party's lawmakers, who overwhelmingly rejected his leadership after Britain's vote to leave the EU last month.

As a result, many of Corbyn’s shadow cabinet members resigned and publicly stated they had no confidence in his leadership.

Corbyn, who still remains the favorite to win, needs to beat Smith in order to keep his post as the opposition leader.

In a boost for Corbyn on Monday, seven leaders from some of Britain’s biggest unions, including Len McCluskey, have written to the Guardian endorsing him as leader and urging their members to support him. 

Smith, a former shadow work and pensions secretary, announced his decision to run in the Labour leadership election last month. Smith had resigned earlier this year from Corbyn's shadow cabinet before challenging for the leadership.

Smith, who pitches himself as the candidate who can “save” the party, faces criticism for his past as a lobbyist for the pharmaceuticals giant Pfizer and his support for the Iraq war.

British opposition Labour Party leadership contender Owen Smith (AFP)

Corbyn’s opponents challenged his leadership for what they call inadequate efforts to keep the UK in the European Union. Nearly 52 percent of British voters made it clear in a referendum on June 23 that they wanted their country out of the bloc.

Corbyn was a fierce opponent of the decision and vouched for a stronger UK role in the EU instead. However, “Corbynistas” argue that Corbyn was going to face an onslaught whatever the outcome of the vote.  

Corbyn has until September 21 to appeal to voters and defeat Smith. The results will be announced in a Liverpool conference three days later.