Labour leader Jeremy Corbyn says having to ask for another Brexit delay was a "diplomatic failure" and another example of ministers' mishandling of Brexit negotiations.
He says the UK will have to prepare to hold European Parliament elections next month, without knowing whether MEPs will take up their seats.
The PM has "stuck rigidly" to a "flawed plan" but the clock has "run down", he adds.
He says cross-party talks are "serious, detailed and ongoing".
He adds that he welcomes indications the government "may be willing to move in key areas".