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LONDON, Oct 24 (Reuters) - Britain's Prime Minister Boris Johnson said on Thursday that he was asking parliament to approve a national election to take place on Dec. 12 as part of his efforts to ensure Britain leaves the European Union.

Johnson said in a letter to opposition Labour leader Jeremy Corbyn that if the European Union offers Britain a Brexit delay to Jan. 31 then he would seek to pass legislation to ratify his exit deal by Nov. 6 and would make available "all possible time" to do so."

This would mean Brexit could be completed before a Dec. 12 election.

"The way to get Brexit done is, I think, to be reasonable to parliament," Johnson said in a televised statement. "If they genuinely want more time to study this excellent deal, they can have it, but they have to agree to a general election on December 12. That's the way forward."

If parliament, which earlier this week rejected Johnson's accelerated timetable for the legislation, did not ratify the deal by Nov. 6, the issue would have to be resolved in the election, Johnson said. (Reporting by William Schomberg and Kate Holton, writing by David Milliken and Kylie MacLellan; editing by Stephen Addison)