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Conservative lawmaker Nicky Morgan dismissed suggestions that Mr. Cameron should step down and said the government had moved to investigate phone hacking with all due speed.

“I think that’s what the Labor Party would love and clearly they’re pushing for it,” Ms. Morgan said. “When you’re in government, it’s very different from being in opposition. It took time to put in place a sufficiently robust inquiry mechanism.”

Mick Hume, a former Times of London columnist, said the growing political crisis is borne of weakness, rather than the actual significance of the scandal itself.

“It does show the weakness of the right, or what we used to call the right,” he said. “They’re chasing to catch up with the illiberal liberals. Whether that translates into votes [at the next election] is another matter. Labor is finding its sense of mission in outrage [rather than policy].”

Mr. Miliband has dismissed calls for statutory regulation of the press but has called for reform. Taking a clear swipe at News Corp., he said his party will submit proposals for new regulations on cross-media ownership.

“The [scandal] has exposed an unhealthy situation involving both of the main political parties, Labor before and the Conservatives afterwards, in the way politicians have courted favor with News International,” said media law consultant David Banks.

“The news organizations are there to hold politicians to account and expose what they are doing. They are not supposed to be in cahoots.”

• Ms. Osborne reported from London, and Mr. Walsh from Belfast.