LONDON - Prime Minister Boris Johnson's Conservatives said on Wednesday they would complete Britain's exit from the European Union by the end of January and hold a budget in February if they win the national election.

With a week to go until the Dec. 12 vote which will decide the fate of Brexit and the world's fifth-largest economy, the Conservatives lead the opposition Labour Party in opinion polls but it remains unclear whether they are far enough ahead to form a majority government.

Setting out their plans for the first 100 days of government, the Conservatives pledged to ratify Britain's EU exit, introduce new domestic law on issues such as health, justice and education, and set out tax cuts in a budget.

"This is the most important election in a generation – important because it will define if we go forward as a country or remain stuck, stalled, repeating the same arguments of the last three years with yet more damaging uncertainty," Johnson said in a statement.

"If there is a Conservative majority next week, we will get Brexit done by the end of January. 2020 will then be the year we finally put behind us the arguments and uncertainty over Brexit."

Labour has promised to renegotiate the Brexit deal and hold a second EU referendum within six months.

If they win a majority, the Conservatives plan to set out their legislative programme on Dec. 19 and then before Christmas bring back the legislation required to ratify Johnson's EU exit agreement and pass it in time to leave by Jan. 31.

The Conservative Party said it would then hold a post-Brexit budget in February "to take advantages of the opportunities provided by leaving the EU with a deal", which would include tax cuts.

Finance minister Sajid Javid postponed a Nov. 6 budget statement due to a delay to Britain's scheduled Oct. 31 EU exit and the government's intention to hold an election. 

(Reporting by Kylie MacLellan; editing by Stephen Addison) ((kylie.maclellan@thomsonreuters.com; +44 207 542 0401; Reuters Messaging: kylie.maclellan.thomsonreuters.com@reuters.net))