Air Force Commander Maj Gen Rumen Radev is among the favourites of the Bulgarian Socialist Party (BSP) for a joint left-wing presidential candidate, daily 24 Chasa quotes high-profile party officials as saying.

Radev is a possible nomination in two out of three options discussed within the BSP, according to the sources.

Socialists held on Saturday their traditional gathering on Mount Buzluzdha, with leader Korneliya Ninova vowing to raise a joint nomination with other left-wing parties in order to defeat any candidate put forward by GERB, the biggest party in the government.

Radev's name was "surprisingly" added by the BSP's headquarters to the list of potential nominees, 24 Chasa quotes the sources as saying as the event was taking place.

The Air Force commander stepped down last year to voice his disagreement with the delay of measures to overhaul the country's defense capabilities.

However, he did not leave the office as Prime Minister Borisov pledged to secure funding.

Possible running mates of Radev include Ivaylo Kalfin, deputy head of left-wing ABV party with which BSP is trying to produce a joint nomination. Another one is Vesela Letcheva, a former sports shooter who is now part of the BSP's leadership after reshuffles made by Ninova in April.

Kalfin is also a possible presidential nominee himself, with Lecheva as a possible running mate, 24 Chasa reports.

The BSP and ABV are now expected to produce a nominee in August.

ABV, however, has already tabled its "recommendation" that its leader, Georgi Parvanov (who served twice as President between 2002 and 2012) should be the joint candidate, while Kalfin should be his running mate.

The BSP has previously rejected the prospect.