The post office will move from its current building in North Road approximately 75m down the street into Garret’s Convenience Store.
The decision follows a six-week public consultation, which closed on Tuesday, June 27, and a review, the spokesman said.
Staff at the Lancing branch were informed today (Tuesday, August 15), the spokesman confirmed.
“Our customers will be offered the same range of services, with more convenient and longer opening hours, including Sundays,” the spokesman said.
“This change is being made as part of the continuing modernisation of our network, and will help us to provide services that will meet customer needs and safeguard future service provision in Lancing, now and for the long-term.
“We are making sure we take the right action to sustain services for years to come.”
A petition against the proposal, started earlier this year by Councillor Lee Cowen, amassed more than 1,500 signatures.
Mr Cowen, who represents Churchill Ward on Lancing Parish Council, said of the news: “It’s disappointing it’s come to this.
“The Post Office hasn’t listened to our concerns.”
But he said this was ‘not through want of trying’ by the community.
“We tried hard to get it to remain in the current building,” he said.
An application made by Lancing Parish Council in February to register the current North Road building as an asset of community value has been rejected, he added.
Tim Loughton, MP for East Worthing and Shoreham, said in a tweet that he was ‘disappointed’ with the news that the Post Office would move.