LONDON (Reuters) - A blueprint for a political transition in Syria, due to be presented by an opposition group in London on Wednesday, offers the first credible picture of a peaceful Syria without President Bashar al-Assad, Britain's Foreign Secretary Boris Johnson said.

The High Negotiations Committee, the main Syrian opposition group at stalled U.N.-mediated peace talks, will propose a gradual transition, starting with six months of negotiations accompanied by a total ceasefire and full humanitarian access.

The plan then foresees the setting up of a transitional administration made up of opposition and government figures which would run Syria for 18 months, during which time Assad would depart. Then there would be elections.

"There is still a chance that this vision can be made to work," Johnson wrote in a column in Wednesday's Times newspaper.

"If the Russians and Americans can together create a ceasefire, then the talks can restart in Geneva with the difference, perhaps, that all sides will by then have seen at least the scaffolding of a post-Assad Syria."

Moscow and Washington are backing opposite sides in the 5-1/2-year-old Syrian conflict, with the Russians fighting on Assad's side while the Americans back opposition groups and insist Assad must go.

The two powers have been negotiating in recent days, with Presidents Barack Obama and Vladimir Putin meeting for 90 minutes on Monday on the sidelines of a G20 meeting in China, but failed to reach an agreement.

However, efforts are still going on behind the scenes and Saudi Arabia's Foreign Minister Adel al-Jubeir said in London on Tuesday that an agreement was possible within 24 hours.

The High Negotiations Committee is backed by Saudi Arabia and Western powers.

In a briefing with British reporters, Al-Jubeir said it was right to pursue the diplomatic effort and "leave no stone unturned" in the quest for a political solution in Syria, but appeared pessimistic. He repeatedly said Assad was not trustworthy and was unlikely to comply with any agreement.

Regional power Iran, arch-enemy of Saudi Arabia, is Assad's strongest supporter and is determined to keep him in power in Damascus.

(Reporting by Estelle Shirbon; editing by Guy Faulconbridge)