Britain must prevent its nationals fighting abroad -report
* Committee worried about Britons being radicalised in Syria
* Terrorist threat to UK rated as "substantial"
* Report raps "weak" parliamentary oversight of spy agencies
LONDON, May 9 (Reuters) - Britain needs to take urgent action to stop citizens travelling to fight in Syria and other conflicts, a parliamentary committee said on Friday, amid fears they could be radicalised there and return to carry out attacks on their homeland.
The head of the Home Affairs Committee also warned that Britain faced a "terrorist threat" as grave as at any point since the Sept. 11 attacks on the United States 13 years ago.
"The number of UK citizens and Westerners travelling to fight in foreign conflicts has reached alarming levels unlike anything seen in recent years," the committee said in a report on counter-terrorism.
"We require an immediate response targeted at dissuading and preventing those who wish to go to fight from going."
Britain and other European governments have stepped up warnings in recent months that Muslims who go to fight against the forces of President Bashar al-Assad in Syria could end up posing a threat to their home countries.
Security chiefs reckon several hundred Britons have joined the Syrian conflict.
Last month, British counter-terrorism police launched a campaign to help Muslim women dissuade young people in their communities from trying to go to Syria.
"Stopping British men and women going to become foreign fighters, in Syria and other theatres of conflict, and engaging with them when they return is vital to avoid endangering the security of the UK for many years to come," said committee chairman Keith Vaz.
Britain has been considered a prime target for militant Islamists since the 9/11 attacks. Four suicide bombers killed 52 people in 2005 when they attacked London's transport system.
Britain's threat level is currently "substantial", the third-highest rating of five, meaning that an attack is a strong possibility, but the committee said the divergent nature of the threat made it more complex than before.
"Recent events involving Boko Haram (in Nigeria), al Shabaab (based in Somalia) and al Qaeda show that the terrorist threat to the UK is as grave as at any point in the past 13 years," said Vaz.
However, the lawmakers on the committee were highly critical of parliament's mechanism for checking on the work of Britain's intelligence agencies following the release of documents by former U.S. National Security Agency contractor Edward Snowden suggesting they were involved in mass surveillance programmes.
The report said the current oversight by parliament's Intelligence and Security Committee (ISC) was ineffective and that this impacted the agencies' credibility.
"It is an embarrassing indictment of our system that some in the media felt compelled to publish leaked information (from Snowden) to ensure matters were heard in parliament," Vaz said.
The comments came a day after the chairman of the ISC, which has seen its powers beefed up in recent years, issued a robust defence of the work of both his committee and of the spies they monitored. (Editing by Gareth Jones)