The current international pressure on Syria is "entirely deserved" and it is now "showtime" for its president, UK Foreign Secretary Jack Straw says.
He said a Syrian official's claims that President Assad had threatened Lebanese Prime Minister Rafik Hariri before his murder were "very serious indeed."
Mr Straw was speaking to the BBC as he started a visit to Lebanon, which has been dominated for decades by Syria.
He said Lebanon was now at a very important crossroads for its future.
A UN investigation into Mr Hariri's death in a massive car bomb in Beirut in February 2005 has implicated Syria, which denies involvement.
On Monday, UN investigators announced they wanted to interview the Syrian president about the assassination. There has been no official response so far.
At the weekend, former Syrian Vice-President Abdul Halim Khaddam alleged President Bashar Assad had made threats to Mr Hariri months before his death.
Mr Assad has pledged to co-operate with the UN investigation, and has allowed some Syrian officials to be questioned.
The murder caused such outrage that Syria was forced to pull all its troops out of Lebanon.
'Really serious questions'
"The pressure on the Syrian regime now is much stronger now than it's been for decades," Mr Straw said.
He said that though the UK and its allies did not encourage or anticipate a collapse of the Syrian government, it was being weakened from within.
The "really serious questions" Syria faced included whether it would co-operate fully with the international community on the investigation into the Hariri assassination, "properly" recognise Lebanon as an independent state and end support for "terrorist" groups and complicity in terrorism.
"So this is showtime for the Syrian president and the regime there," Mr Straw said.
"The onus is now on the Syrian regime to match the expectations which were raised by President Bashar when he first took over from his father... and break away from this long legacy of failing to meet the requirements of international law and what appears to have been complicity in some very bad things that have happened in the Lebanon."
He urged the Syrian president to "start implementing fully UN resolutions" and co-operate with the Hariri probe straight away.
Mr Straw - the highest-ranking British official to visit since 1998 - is having talks with Prime Minister Fouad Siniora and is expected to tell him Britain supports Lebanon's efforts to implement political and economic reforms.
Mr Siniora's government has been in crisis for several weeks since five pro-Syrian cabinet ministers decided to boycott cabinet sessions.
The UK foreign secretary will also meet the Christian Maronite patriarch and the spiritual leader of Lebanon's Shia Muslims.