Royal Bank of Scotland said on Thursday “there is a significant risk” that it will miss the December 31, 2017 deadline for it to sell its Williams & Glyn branch network — a condition of its £45.5 billion bail out by the UK taxpayer.

RBS shares fell as much as 5%.

“Since the last update provided with the 2015 annual results, we have undertaken further extensive analysis on the separation and divestment of Williams & Glyn,” said RBS.

“As a result of this analysis, we have concluded that there is a significant risk that the separation and divestment to which we are committed will not be achieved by 31st December 2017.

“Due to the complexities of Williams & Glyn’s customer and product mix, the programme to create a cloned banking platform continues to be very challenging and the timetable to achieve separation is uncertain.

“RBS is exploring alternative means to achieve separation and divestment. The overall financial impact on RBS is now likely to be significantly greater than previously estimated.”

RBS, still 73% owned by the UK taxpayer, agreed to sell the Williams & Glyn network before the end of 2017 under the terms of its bailout during the 2008 financial crisis.

A spun-off Williams & Glyn would be expected to become one of the UK’s most prominent “challenger banks.”

RBS is due to announce first-quarter 2016 results on Friday, April 29.

In December, RBS said it had received “a number of informal approaches” for the Williams & Glyn network.

But at that time, RBS said that it would also continue preparations for an initial public offering (IPO) of Williams & Glyn and that it had submitted a banking licence application for the network.

In 2013, RBS raised roughly £600 million from a group of investors including Corsair Capital, Centerbridge Partners and the Church of England’s investment fund for a stake of up to 49% in Williams & Glyn.