The UK Justice Secretary, Chris Grayling, says the prison which will be built in north Wales will be 'Britain's biggest'.
He says the announcement is a very important part of upgrading Britain's prisons.
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Horizon Nuclear Power says it is presenting a more compact design. It says the project represents a multibillion pound investment.
After a cloudy start, there'll be some bright or sunny spells developing throughout the day. It'll feel warmer than yesterday.
Prime Minister Theresa May announced the decision from Downing Street.