MOSCOW, June 22 (Reuters) - State prosecutors in Belarus asked a court on Tuesday to hand down a 15-year jail term to a former banker who was taken into custody last year after challenging President Alexander Lukashenko at an election, the TASS news agency reported. Viktor Babariko, the former head of Belgazprombank, is being tried on corruption charges that his team says were fabricated to prevent him from standing as a candidate in last August"s presidential election. The election that handed Lukashenko a new term as president sparked mass protests over allegations of vote rigging that the authorities denied.