Boris Johnson and his girlfriend Carrie Symonds could become the first unmarried couple to officially live in Number 10.
The couple has been living together following 55-year-old Mr Johnson's split with his wife Marina Wheeler.
A source close to the new leader of the Conservative Party confirmed on the weekend that Ms Symonds, 31, is expected to move into Downing Street when he becomes prime minister.
It came after the Sun quoted a friend telling the paper: "They've been living together a while and have agreed it will carry on if he wins on Tuesday."
They would be the first unmarried couple to officially live in Number 10.
The couple had been living at Ms Symonds's flat in Camberwell, south London, until a well-publicised row recorded by neighbours last month, and have reportedly since bought a four-bedroom property nearby.
Mr Johnson, who is the MP for Uxbridge and South Ruislip, announced last autumn that he and his wife were divorcing after 25 years of marriage.
The couple said the decision had been taken some months earlier and the announcement came after newspaper allegations about Mr Johnson having had another extra-marital affair.
Public relations expert Ms Symonds joined the Tory party media machine in 2009, first as a press adviser, then as head of broadcast at Conservative Campaign Headquarters ahead of the 2015 general election. She was made head of party PR aged just 29.
Her association with Mr Johnson dates back to early in her career, having worked on his successful re-election bid at City Hall in 2012.
Today she is a senior adviser at Oceana, a US-based environmental campaign group, working with its marketing operation in London.
A profile on the charity's site describes her as being "passionate about protecting the oceans and marine life".