Liberal Democrats must show voters they can trump their rivals' law and order policies by proving that Labour and Tory rhetoric masks a failure to curb crime, Mark Oaten will tell his party's spring conference tomorrow. "Tough slogans are soft options," he says.

In a pre-conference interview with the Guardian the home affairs spokesman admitted that accepting Labour's plans for control orders to tackle terrorism was "not a popular decision" with all his colleagues. "There will be some cases where you cannot prosecute," he said. "As a mature party - a sensible party - we cannot just walk away from it."