(Reuters) - Prime Minister David Cameron will announce plans on Wednesday to tighten the law to help stop charities being used as a front to raise funds for terrorist groups.

Several individuals convicted of terrorism offences in Britain had raised funds in public, purportedly for charitable purposes, the majority of which the charities never received, the government said.

Three British Islamists jailed last year for planning mass suicide attacks had tried to fund their plot by posing as street collectors for the Muslim Aid charity organization, raising 12,000 pounds.

The planned new powers include banning those who have criminal convictions, such as for terrorism offences or money laundering, from being a charity trustee.

The Charities Commission, a regulatory body which earlier this year asked the government for more funding and powers to tackle abuse in the sector, will be given the authority to disqualify trustees it considers unfit and to shut down a charity where there has been mismanagement.

"I want us to confront the menace of extremism and those who want to tear us apart," Cameron said in a statement. "Today's changes will help make sure that when people donate to charity, their money always goes to genuinely good causes."

Cameron, due to chair a meeting of the government's Extremism Taskforce on Wednesday, will also announce an extra 8 million pounds of funding for the Charity Commission to help it combat a range of abuses including terror financing, tax avoidance and fraud.