It comes days after the surprise departure of Boris Johnson's chief adviser on the union.
Read moreDavid Frost
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Video caption: Lord Frost calls for 'different spirit' from Brussels The UK's chief Brexit negotiator says the relationship with the EU has been "problematic" post-Brexit.
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Video caption: Lee Cain: Public 'looks on with astonishment', says Labour's Reeves Labour's shadow minister criticised the government for "focusing on internal battles" in Downing Street.