The Prime Minister said he was "deeply concerned" about the case of Mohammad Asghar, 69, from Edinburgh.
His comments yesterday in the Commons came just hours before Foreign Office representatives were due to meet with staff at the Pakistan High Commission in London.
Mr Cameron said the UK Government was doing everything in its power to express its opposition to the death sentence.
In answer to a question from Edinburgh East Labour MP Sheila Gilore at Prime Minister's Questions, he said: "I too am deeply concerned about this death sentence passed on to Mr Mohammad Asghar and, as you know, it's our long-standing policy to oppose the death penalty in all circumstances.
"The Pakistani authorities can be of no doubt of the seriousness with which we view these developments."
He said Foreign Office Minister Baroness Warsi had spoken to the Chief Minister of the Punjab on Monday and that the High Commissioner in Islamabad is continuing to raise the case with the authorities there.
He added that Foreign Office officials are meeting Pakistan High Commission officials in London today to discuss his and other cases.
Mr Cameron added: "We take this extremely seriously and we're making that clear at every level."
Ms Gilmore had told the Prime Minister that Mr Asghar had been diagnosed with paranoid schizophrenia in 2010 and had been treated for the condition in Edinburgh "but the judges refused to take that into account".
Mr Asghar received the death penalty after he wrote letters in which he claimed to be the Prophet Mohammad. The death sentence against the elderly Briton, who returned to Pakistan in 2010, was delivered late last week.
However, questions have been raised over Mr Asghar's trial after it emerged that lawyers were blocked from visiting him on numerous occasions.
Meanwhile, the Moderator of the General Assembly of the Church of Scotland has called on Foreign Secretary William Hague to raise the issue of blasphemy laws with the Pakistani authorities.
The Right Reverend Lorna Hood has written to Mr Hague over the Church's concerns about a recent ruling from Pakistan's Federal Shariah Court (FSC), saying the death penalty should be the only punishment for anyone convicted of blasphemy.
Rev Hood says that the Church fears that the law will be used to persecute non-muslims and those with minority beliefs in Pakistan.
Her letter sates: "Christians and other non-Muslims would face restrictions and thus achieving justice could be more difficult.
"The Church of Scotland fears that if the FSC order is implemented the misuse of blasphemy laws which, we already know and even some of the Pakistani politicians have admitted, are being used to settle personal scores, will increase and victims from minority faith communities will become even more vulnerable than at present."