LONDON (Reuters) - Opposition Labor Party leader Jeremy Corbyn said on Friday he had a secret government report revealing that there would be tariffs between Northern Ireland and the mainland under the Brexit deal negotiated by Prime Minister Boris Johnson.

"This is the shocking and definitive evidence that categorically shows the detrimental impact of Johnson's exit agreement on large parts of our country," Corbin said in a speech in London.

The main message of Johnson's campaign ahead of next week's election is that he will 'implement the exit deal' three years after the 2016 referendum, but Corbin said the slogan was misleading.

He said the prime minister wanted the British to vote without seeing things as they were, adding that the leaked government document revealed there would be customs duties and security checkpoints between Britain and Northern Ireland, adding that this contradicted Johnson's claim that there would be no border in the Irish Sea.

The Labor plan to leave the EU would negotiate a new deal involving a customs union and then hold another referendum on whether to approve the deal, although Corbin said he would remain neutral.

"After the decision is made, we will need a prime minister who is able to talk to both sides and unite the country," Corbin said. "I think we currently have enough reasons for division," he said.