Britain's major parties have launched their election campaigns in the run-up to the Dec 12 vote by vying with one another in promises to resolve the dispute over Brexit.

But while the ruling Conservatives and their chief opponent Labour both vowed to make quick progress on Brexit, their first skirmishes in what promises to be a gruelling and ill-tempered campaign offered no fresh ideas on how Britain can emerge from the morass.

Prime Minister Boris Johnson's Conservatives have the simpler message: His government last month negotiated a deal governing the terms of Britain's departure from the European Union.

Mr Johnson promises that if he wins a majority on Dec 12, the new Parliament would quickly ratify the deal and Britain would be out of the EU by January.

"The choice before the country is really very clear," he told the electorate after the last full Cabinet meeting of his government.

"Do you want to go forward with our agenda, which is to get Brexit done, and then get on with delivering all the wonderful things we want to do for this country... or do you want to waste 2020?"

But Mr Johnson quickly came under fire for his inability to say what would happen to Britain's trade relations after leaving the EU.

The Brexit deal he negotiated merely covers the terms of the departure; ahead lies a much more difficult negotiation about the permanent trade relationship between Britain and Europe.

On the hustings, Mr Johnson claimed he would conclude a free trade deal with the EU by January 2021, which coincides with the end of the transition period Britain has to disengage its affairs from Europe.

But Labour dismissed this timetable as "fanciful", pointing out that the trade deal between the EU and Canada - which many Conservatives want to copy for Britain's own treaty with Europe - took seven years to complete.

The danger, Labour claims, is that unless Mr Johnson gets another extension for the negotiations with Europe - something the Prime Minister rules out - Britain could still crash out of the EU with no permanent trade pact.

Yet, Labour's position on Brexit is also coming under intense scrutiny, given that many of the party's traditional supporters voted to leave in the 2016 referendum; some of the highest pro-Brexit majorities were in solidly Labour constituencies.

But party leader Jeremy Corbyn, eager to retain the loyalty of many Labour MPs who support staying in the EU, is offering an alternative compromise solution that risks pleasing nobody.

He promises that if he wins the polls next month, he could strike a "sensible" new deal with the EU within months "based on terms we have already discussed", such as Britain's membership of a Customs union with Europe.

But at the same time, Mr Corbyn says a Labour government would plan for a new referendum in June or July next year, with the electorate asked to either approve the deal or decide that Britain should remain a member of the EU after all.

"Labour's plan would get Brexit sorted so a Labour government can get on with delivering the real change Britain needs," he told activists on the hustings.

But when faced with a barrage of questions, Mr Corbyn could not explain why the EU would consent to another round of negotiations with the British, especially since European officials would be aware that whatever they negotiate with a Britain ruled by Mr Corbyn's Labour would have to be put to the electorate for a referendum.

More confusingly still, Labour leaders who support the EU have admitted that if the party gains power and renegotiates terms with the EU, they would campaign in a subsequent referendum to reject the deal which they negotiated.

In effect, therefore, they propose to sign a deal they do not like, in the hope that it would be rejected.

The Conservatives hit hard.

"It is a fairy tale if you imagine Jeremy Corbyn can get Brexit done. His policy on Brexit has been constructed by a terminally weak leader in order to paper over the cracks in his party," claimed Mr Michael Gove, the minister in charge of Brexit planning in the Conservative government.

Both parties know these early clashes are merely a prelude to the bigger battle.

And while opinion polls indicate that the Conservatives have the support of around 36 per cent against 25 per cent for Labour, with much of the rest split among smaller parties, few will predict the election outcome.