Video report by ITV News political correspondent Romilly Weeks

David Cameron has backed Barack Obama after the US president announced he is seeking congressional support to launch military action against Syria.

American military chiefs stand ready to act tomorrow, the president said, but going to a vote effectively rules out intervention until Congress returns to session on Monday, September 9.

Before then, the US president, Mr Cameron and other international leaders will gather in St Petersburg for the G20, hosted by Russian president Vladimir Putin, a key ally of the Syrian regime.

Pictures: Stop the War Coalition marches in London over Syria

In a move that echoes the Prime Minister's approach, President Obama said he will attempt to seek a domestic political mandate for action despite opposition from advisers, who pointed to the dangers of such a move in light of Thursday's parliamentary defeat.

Mr Cameron quickly took to Twitter to endorse the president's approach, writing:

Foreign Secretary William Hague followed, tweeting:

Former Liberal Democrat leader Lord Ashdown raised the prospect of British support being put back on the table, telling the BBC parliament could "reconsider its position".

"This was a brave and principled act from a brave and principled president," he added.