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UPDATE 1-UK's Labour will try to amend Brexit deal legislation

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LONDON, Oct 20 (Reuters) - Britain's opposition Labour Party will try to amend legislation to ratify Prime Minister Boris Johnson's Brexit deal, the party's finance spokesman John McDonnell said on Sunday.

Johnson has said he will bring forward the legislation in parliament next week after parliament voted on Saturday to withhold approval of the agreement until formal ratification legislation has passed.

"We'll be moving amendments and see whether or not we can get some form of agreement through the House of Commons that's then acceptable," McDonnell told Sky News, citing issues Labour wants to see included such as protection of workers' rights.

He also said the issue of a second Brexit referendum would "almost inevitably come up", but that any amendment proposing one would be more widely accepted by lawmakers from other parties if it did not come from Labour's leadership team.

"We've always said actually let the people decide," he said.

McDonnell said Johnson's decision to send an unsigned letter to the EU requesting a delay to Brexit as well as another note in which he explained that he did not want a "deeply corrosive" extension could put him in contempt of court.

Johnson was compelled to write the letter by a law passed last month by opponents and his government made undertakings to a Scottish court that it would not seek to frustrate the legislation.

"He may well be in contempt of parliament or the courts themselves because he's clearly trying to undermine the first letter," McDonnell said.

"And not signing the letter, he's behaving a bit like a spoiled brat, parliament made a decision, he should abide by it." (Reporting by Kylie MacLellan and Paul Sandle; Editing by Dale Hudson and Jane Merriman)