Former Conservative MP Sam Gyimah who backed calls for a second referendum has joined the Liberal Democrats.
Six MPs have defected to the party in recent weeks, including former Tory MP Philip Lee, and ex-Labour MPs Luciana Berger and Chuka Umunna.
Mr Gyimah was one of the 21 Tories who had the Conservative whip removed after rebelling against Boris Johnson in a bid to prevent a no-deal Brexit.
The 43-year-old briefly stood in the race to become Conservative Party leader after Mrs May resigned.
The Lib Dems currently now have 18 MPs, having been boosted by a victory in the Brecon and Radnorshire by-election and the defections.