India visit: 'Plain speaking' Cameron stands by his comments on Pakistan

The Prime Minister ignored calls from Pakistan's high commissioner to London for him to "make amends" and repeated that Islamabad must "crack down" on terrorist groups.

But he stopped short of his provocative claim that Pakistan must not "export terror to the world" or be allowed to "look both ways" on the issue.

Apparently signalling a new era of straight-talking diplomacy, Mr Cameron said: "I believe in speaking clearly and plainly about these matters and we have seen not just the threat of terrorism but the reality of terrorism in the enormous losses that we saw on the streets of Mumbai, that we have seen on the streets of London, and that we see week after week in Afghanistan.

"It is not acceptable, as I have said, for there to be within Pakistan the existence of terror groups that cause terrorism both within Pakistan and also outside Pakistan, in Afghanistan, in India and elsewhere in our world."

But his latest comments, at a joint Press conference with Indian Prime Minister Manmohan Singh at the end of a visit to New Delhi, were not as strong as those he made off-the-cuff on Wednesday.

They caused Pakistan's high commissioner in London, Wajid Shamsul Hasan, to say that people in his country were "really hurt".

Foreign Secretary William Hague insisted the Prime Minister was "a great diplomat... a natural".

But the PM faces a potentially difficult meeting with Pakistan's president, Asif Ali Zardari, when he visits Chequers next week.