LONDON - Britain’s failure to pass a law forcing communications firms to collect and store online data is endangering lives and weakening the country’s ability to protect itself, Theresa May, the interior minister, warned on Wednesday.

‘Every day that passes without the proposals in the communications data bill, the capabilities of the people who keep us safe diminishes and ... more people find themselves in danger and yes crimes will go unpunished and innocent lives will be put at risk,’ May, the home secretary, told parliament.

May, a member of Prime Minister David Cameron’s Conservatives, said the extra capabilities were urgently needed, but that party political differences were blocking legislation. The Liberal Democrats, Cameron’s junior coalition partner, oppose the move, saying it would infringe people’s privacy.

The head of Britain’s GCHQ eavesdropping agency last year called on technology firms Twitter Inc. and Facebook Inc to allow security services greater access to their networks, citing their huge importance to militant groups.

The head of Britain’s MI5 Security Service has echoed that concern over lack of access to communications, warning last week that Al Qaeda militants in Syria were plotting attacks to inflict mass casualties in the West. Cameron has said if he is re-elected in a national vote in four months time, he will bring in new laws to give security and intelligence services greater powers to monitor Internet communications.