The government said the funding, the largest to date, would allow early construction work to continue at the plant being built by French energy giant EDF before a final investment decision expected later this year.
The fresh money, which the government said was made available from existing budgets, comes in addition to a 700 million-pound funding pledge in 2022, as well as a further 511 million pounds agreed last summer.
"It's a win for our energy security and sends a strong message to investors that Britain is serious about its low-carbon, homegrown nuclear-powered future, providing reliable, cheaper power for British families," Nuclear Minister Andrew Bowie said in the statement.
Last September the government opened the search for private investment in the plant, which is expected to produce around 3.2 gigawatts of electricity, enough to power around 6 million homes for over 60 years.
The Russia-Ukraine war, which had sent energy prices soaring in 2022, had caused major countries around the world to rethink their energy security.
Currently, about 15% of Britain's energy needs are met by nuclear power, a drop from 27% in the 1990s as older plants have been decommissioned and new ones are taking time to build.
