While Theresa May faces an uphill struggle to win parliamentary backing for her EU withdrawal deal this month, the leader of the opposition Labour Party, Jeremy Corbyn, is likely to face a defining moment of his own in relation to Brexit.
At the Labour Party conference last September, the Corbyn leadership came under severe internal pressure to embrace a second referendum on Britain's membership of the EU.
An apparent compromise was reached. If May's EU withdrawal deal failed Labour's six "tests" for acceptance, the Corbyn leadership would press for a general election and if that option was not available, it would then support a People's Vote or second referendum.
However, Corbyn has recently made it plain that Brexit would go ahead if May improved negotiated terms with Brussels or even if his party won a snap general election sometime in 2019.
But for many people in the British Labour Party, Corbyn's position seems increasingly delusional.
First, the EU referendum of June 2016 did not provide a definitive and lasting mandate for Brexit. It is contestable to claim that a narrow 3.8 per cent margin of victory for the Leave camp in a non-binding referendum politically justified the greatest change in British external policy since World War II.
It is the hallmark of a democratic society that dissent is not only tolerated but is part and parcel of political debate, and that political decisions are reversible.
There is no suggestion, for example, the Labour leadership views the Conservatives' election victory of June 8, 2017 as an irreversible event. Yet, strangely enough, that is exactly what Corbyn seems to be saying about the 2016 referendum.
Moreover, the integrity of the EU referendum result has been tainted by revelations concerning the use of illegal money, data crime, and possible Russian interference, and mounting evidence that many of the promises by the Leave camp, such as huge funding increases for the NHS and significant reductions in immigration, have turned out to be largely false.
Second, Corbyn seems attached to the mythical idea that Brexit will help the UK "take back control".
Like it or not, globalisation has redefined state sovereignty. Today, all states are confronted by economic, security and environmental challenges that do not respect territorial boundaries.
Corbyn may dream (like May) of dictating the terms of Brexit negotiations, but the reality is that Britain ultimately will have to accept the terms handed down by the other 27 independent liberal democracies that comprise the EU.
Slogans like a "jobs Brexit" implies that Labour can somehow build socialism in one country after leaving the EU, but such aspirations are divorced from the realities of an interconnected world.
Britain is a middle-size market and when it is no longer part of a much bigger EU single market, London will struggle to attract comparable levels of external investment and trade.
Third, many in Labour's ranks are demanding Corbyn decisively break with the hard-right economic agenda of the Brexit leadership.
Supported by free-market libertarians like Peter Hargreaves and Arron Banks, and backed by a cabal of media billionaire cheerleaders like Rupert Murdoch, Richard Desmond, Viscount Rothermere, Sir David Barclay and Sir Frederick Barclay, the main goal of these Brexit backers is to make Britain into a Singapore-style low-wage, low-benefit and low-regulation economy.
When Brexit leaders refer to casting off the burdens of EU regulations, they are talking about removing the UK from guaranteed workplace standards - paid holidays, protection against discrimination, and paternity leave for parents. The Brexit leaders believe this economic strategy and a new freedom to negotiate free trade deals will kickstart the British economy.
But the UK's move to isolate itself from the most prosperous tariff-free market of 520 million consumers in the world looks like bad news for the mass of the British population which are already seeing their prosperity, security and their liberty significantly decline.
Meanwhile, increasing numbers of national and international firms in London continue shifting their premises to other European countries with the loss of thousands of jobs in the UK.
Unless Corbyn firmly opposes Brexit - a project that clearly seeks to advance the interests of the few at the expense of the many - his dreams of becoming the UK's next Labour prime minister are unlikely to be realised.
• Robert G. Patman is a Professor of International Relations at the University of Otago, New Zealand.