“The discussions with Labour have been serious but had also been difficult in some areas, such as in relation to the timetable for the negotiations,” the prime minister told her cabinet.
The criticism is striking because it is the first time the government has turned on Labour since approaching the opposition three weeks ago.
Until now, it has been Mr Corbyn’s party that has blamed the prime minister for the lack of progress, accusing her of refusing to shift on her red lines – in particular, membership of a customs union.
The cabinet discussion took place even before the talks resumed on Tuesday afternoon – but after a failure to hold any meaningful negotiations last week.
Ms May’s spokesman told journalists: “The PM said the discussions with Labour had been serious, but had also been difficult in some areas, such as in relation to the timetable for the negotiations.
“The PM said that the government’s position was that progress needed to be made urgently as it was vital to deliver on the result of the referendum and for the UK to leave the European Union as soon as possible.”
Asked if the reference to difficulties being caused by the “timetable” referred to Labour not wanting them to conclude as quickly as the government wanted, the spokesman said such an interpretation was “reasonable”.
But he rejected a suggestion that meant Labour was guilty of “bad faith”, adding: “Labour has approached [the talks] in a serious manner and has been engaging constructively.”
Downing Street is increasingly gloomy about the chances of a breakthrough, given there is no obvious reason for Mr Corbyn to help the prime minister escape the Brexit crisis.
But it appears impossible for the prime minister to concede a customs union, which would split her party and could hasten her ejection from Downing Street.
One possibility is to bring forward the withdrawal agreement bill as early as next week, to keep open the slim possibility of full ratification by 22 May.
“It’s a piece of legislation that is required,” Ms May’s spokesman said. “We have been working on the withdrawal agreement bill for a significant amount of time.”
However, No 10 knows that, if the bill was thrown out – a strong probability, without a deal with Labour – it would be forced to prorogue parliament and start a new session, threatening the fracturing alliance with the Democratic Unionist Party.