UPDATE 2-UK minister forced out in blow for Johnson as election campaign starts

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LONDON, Nov 6 (Reuters) - British Prime Minister Boris Johnson's minister for Wales Alun Cairns resigned on Wednesday, the first official day of the general election campaign, after being accused of lying about his knowledge of an aide who allegedly sabotaged a rape trial.

Cairns had been under pressure to resign after insisting last week he was unaware of evidence given by his former aide which led to the collapse of a trial in April last year.

However, the BBC reported this week that Cairns was emailed about the matter in August 2018 and later endorsed the aide as a candidate for the devolved Welsh assembly four months later.

Rival political parties had been calling for Cairns to quit. The Labour Party’s shadow minister for Wales, Christina Rees, said Cairns had been caught “brazenly lying”.

The row had threatened to derail the Conservative campaign in Wales which contains key seats that Johnson would need to win to secure a parliamentary majority.

Cairns sent a letter to Johnson saying he was confident he would be cleared by any investigation but was stepping down because of the sensitivity of the allegations.

"This is a very sensitive matter and in the light of continued speculation, I write to tender my resignation as Secretary of State," Cairns said in a letter to Johnson.

Johnson thanked Cairns for his work as minister.

The Conservative campaign had already started badly before Cairns' resignation.

On Tuesday, minister Jacob Rees-Mogg was forced to apologise for implying that victims of the blaze at London's Grenfell Tower, which killed 71 people, should have used common sense to ignore firefighters' instructions to stay in the burning building until help arrived.

On Wednesday, the party was accused of putting out a doctored video clip of a television interview with a senior Labour politician.

(Reporting by Andrew MacAskill and Kate Holton; editing by Stephen Addison)