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Crossrail bosses are expected to announce next Thursday when they think the new line will open.

Facing questions from the London Assembly鈥檚 transport committee earlier today, Transport for London chief Mike Brown said he, along with Crossrail鈥檚 chief executive Mark Wild, its chairman Tony Meggs as well as London mayor Sadiq Khan were preparing to explain the new plan.

He said: "The mayor has written to the chair of the Assembly offering a briefing from [the four] as soon as we have a clear and indicative plan of the new schedule emerging."

At this morning鈥檚 meeting, committee chair Caroline Pidgeon said she understood the briefing looked likely to take place a week today (2 May).

Wild has previously said a new opening window would be made public by the end of April, admitting Crossrail won鈥檛 open at all this year.

Reports before Easter suggested Crossrail might not be ready until spring 2021 鈥� nearly two and a half years after it was supposed to open.

Sources told the BBC that testing was 鈥減roving more difficult than was first thought鈥�, saying: 鈥淚t all depends on how dynamic testing goes between now and the end of this year.

鈥淭he last quarter of this year will be a critical period for the testing.鈥�

The source said a 鈥渂est case scenario鈥� would see Crossrail open in spring 2020, while a 鈥渕iddle probability case鈥� would be the summer of the same year. The source said: 鈥淎 worst case is the spring of 2021.鈥�

In a letter to Pidgeon sent at the start of the month, Wild revealed that close-headway testing 鈥� which tests running trains close together 鈥� had fallen behind.

He said the testing, which was due to start more than five weeks' ago, had been pushed back until after Easter due to a 鈥渟afety critical failure鈥�. In his letter to Pidgeon, Wild did not spell out what this failure was.