The government and French energy company EDF are expected to sign the key contract for the new £18 billion Hinkley Point C nuclear power station.
With the government giving the go-ahead for the plant, which will power nearly six million homes, Hinkley Point C will become the UK's first new nuclear plant in a generation.
Jean-Bernard Levy, EDF CEO, is now expected to join officials from the UK, France and China at the behind-closed-doors ceremony in London, in a second attempt at finalising the deal after Prime Minister Theresa May unexpectedly announced in July that she needed more time.
The key document is the Contract for Difference, or CfD, which gives a guaranteed price for the electricity Hinkley will generate for 35 years. In return EDF, with its Chinese partner CGN, will finance the project and shoulder the risk of any delays.
The Department for Business is expected to confirm the formal signing on Thursday afternoon (29 September).