(Changes date, adds context, details)
LONDON, Sept 1 (Reuters) - Prime Minister David Cameron will announce new laws on Monday to try to stop radicalised Britons returning from Syria and Iraq launching attacks on British soil, after a video purportedly showed a London-accented man beheading a U.S. journalist.
The announcement, expected around 1430 GMT, comes after Cameron raised Britain's terrorism alert to its second-highest level last week saying Islamic State (IS) in Syria and Iraq posed the country's greatest ever security risk.
Ministers from Cameron's Conservative party say the new laws are needed to beef up Britain's defences against the threat that those who have been radicalised and fought alongside Islamic extremists could return home with violent intent.
Hours before its announcement, the package of measures had yet to be finalised, with the junior partner in Britain's two-party coalition government wary of bringing in new laws that could limit civil liberties.
The response comes after IS released a video in August showing the killing of U.S. reporter James Foley, prompting a manhunt by British security services and renewed concern about Britons fighting in the region.
Among the new measures reported to be under consideration is a temporary ban on allowing the re-entry of British Islamists who have fought in Iraq or Syria. Airlines may also be obliged to share more data about passengers.
British nationals could be allowed to keep their citizenship but would be prevented from re-entering the country for a period of time, the BBC said, citing a government source.
Previously, Britain only had powers to prevent foreign nationals, naturalised citizens and those with dual nationalities from returning, the source said.
The government is also looking at ways to make it harder for potential fighters to travel abroad by making it easier for authorities to remove their passports through additional temporary seizure powers at the border.
Edward Garnier, a Conservative lawmaker who was formerly one of the government's top legal advisers, warned that the new package of measures would have to comply with international laws and was wary it might duplicate existing powers.
"I don't complain about the government trying to get its head round this issue," he told BBC radio. "What we need to be careful of in parliament is that we don't just say and do things because it sounds good, when we could do those things already." (Reporting by Aashika Jain in Bangalore and William James and Sarah Young in London; Editing by Andrew Osborn)
Our Standards: The Thomson Reuters Trust Principles.