British Airways said its passengers now have access to many more routes since it added its flight code to more than 170 Vueling flights.
It hailed the codeshare as "a significant move" for the two IAG carriers, saying it heralded the beginning of a closer relationship.
More codeshare routes are planned for the future, it said.
British Airways customers will be able to book Vueling flights through all BA sales channels, including ba.com, and collect Avios on these bookings.
The codeshare routes are largely centred on Vueling’s operation in Italy with 37 international and 11 domestic routes available from Vueling’s Rome Fiumicino base.
These include Brindisi, Palermo, Lamezia, Valencia, Split and Nantes, none of which are served directly by BA.
Other new routes on offer through the codeshare include Heathrow to Bilbao and La Coruña, Cardiff to Malaga and Alicante, and Edinburgh to Barcelona.
The codeshare routes are on sale now for travel from June 24, 2014.
