By Andrew Osborn, Paul Taylor and Guy Faulconbridge
LONDON (Reuters) - Britain has abandoned plans to arm Syrian rebels fighting to overthrow President Bashar al-Assad and believes he might survive in office for years, sources familiar with government thinking say.
The sources also told Reuters that a peace conference to try to end the conflict - now in its third year - might not happen until next year if at all.
"Britain is clearly not going to arm the rebels in any way, shape or form," said one source, pointing to a parliamentary motion passed last week urging prior consultation of lawmakers.
The reasons for the shift were that British public opinion was largely opposed, and there were fears that any weapons Britain supplied could fall into the hands of Islamist militants.
"It will train them, give them tactical advice and intelligence, teach them command and control. But public opinion, like it or not, is against intervention."
The British position amounted to one of the gloomiest assessments of the rebels' prospects yet.
It was Prime Minister David Cameron who led the charge earlier this year for the European Union to drop an arms embargo on Syria, which London and Paris had argued was one-sidedly penalizing the anti-Assad opposition.
The involvement of Iran and Hezbollah had shifted the balance of power on the battlefield in Assad's favor, the sources said, giving him less incentive to negotiate, and the West had no strategy to end the conflict soon.
"The Western assessment has changed," said one source. "We thought Assad could only hold on for a few months. We now think he can last a few years."
Hobbled by debt and defense budget cuts at a time when the United States, Britain and NATO allies are withdrawing forces from Afghanistan, the West says it wants to help the rebels topple Assad.
But it finds its options limited.
Forces loyal to Assad have made gains in recent months, while rebel groups have been plagued by infighting between Islamist militants linked to Al Qaeda and the more moderate Free Syrian Army. The longer the conflict drags on, the greater the influence the West thinks the Islamists will have, the sources said.
U.S. efforts to arm the rebels have stalled in Congress. Britain publicly says it is not ruling out arming the opposition but has privately done so, the sources said.
Only a dramatic shift in the situation such as "widespread use of chemical weapons" might force a rethink, another source said, refusing to be drawn on whether a collapse in attempts to broker a political solution might also be a tipping point.
Britain has said it believes there has so far been only small-scale use of chemical weapons.
Political realities mean any British decision to arm the rebels would need to be endorsed by a vote in parliament anyway.
John Baron, a lawmaker from Cameron's ruling Conservatives who tabled the motion requiring the government to give parliament such a vote, said he thought a majority of lawmakers opposed sending weapons to the rebels. The government would lose such a vote if one were held today, he said.
He said the vote may have been one factor which persuaded the government to back away from arming the opposition.
"The foreign secretary has formally said that no options are off the table," he said. "But what we do know is that Number 10 (the prime minister's office) has been keen to explore the possibility of arming the rebels."
Any move to arm the rebels would increase the violence and suffering, he said, adding the government had not adequately explained how it would stop arms from falling into the hands of extremists on the rebel side.
While Gulf allies like Saudi Arabia might be willing to supply arms to the rebels, and France might quietly channel some weapons, the first source said they were unlikely to provide "game-changing" weapons.
"If you give arms to General (Salim) Idriss, how sure can you be that they won't end up in the wrong hands and be used to shoot down a Western civilian airliner?" the source said.
France's foreign minister said on Thursday Paris had still not decided whether to arm the rebels.
"There are certain conditions that need to be met before eventually sending weapons," Laurent Fabius told reporters. "For now France has not modified its position. We have the ability to do it, but we haven't delivered any lethal weapons."
The newly-elected president of the Syrian National Coalition Ahmad Jarba is due in Paris for talks over the next week.
NO SILVER BULLET
The West's efforts to end the conflict were being pursued "on different tracks", added another source.
But nothing was happening on any of them.
"If there is a silver bullet we don't know what it is," the source said. "All the options are horrific."
Sources said Britain was unwilling to publicly disavow the option of arming the rebels because keeping the option open might help persuade Assad to sit down for peace talks.
"None of us can foresee exactly how this crisis will develop," British Foreign Secretary William Hague told lawmakers this week. "We have taken no decision, but we have not ruled anything out."
Russia - Assad's most powerful foreign backer - and the United States want peace talks in Geneva to try to agree a ceasefire and the makeup of a transitional government. The sources said the initiative had stalled and there was a risk such talks would never happen.
"We'd hope they would happen this year," said one. "It's an aim that's realistic. But it could be next year. It's been drifting. We can't drag it out indefinitely."
Imposing a no-fly zone was a non-starter. "It would need boots on the ground to enforce it and would be horrendously expensive," the source said.
Iran's decision to pour money and men into Syria to support Assad had made things even harder, another source said, adding there were perhaps as many as 10,000 Lebanese Hezbollah guerrillas and Iranian fighters inside Syria including members of the elite Qods brigade of the Revolutionary Guards.
Hezbollah openly boasts of its presence in Syria but Iran denies it has troops on the ground.
"Assad cannot score a decisive victory, but he can score tactical victories, as he did in Qusair and as he may do in Homs," said the first source.
"The opposition still controls significant swathes of territory along the borders that he cannot recover, and they can hop over the border and regroup."
(Additional reporting by Mark Hosenball in Washington and John Irish in Paris; Editing by Andrew Roche)