Tony Blair is in for a difficult few weeks in the run up to next month's elections, Jack Cunningham has said.
Is Tony Blair facing his toughest test as prime minister?
But the prime minister's former cabinet enforcer sought to play down reports of a growing restlessness within the Parliamentary Labour Party.
However BBC political editor Andrew Marr says Mr Blair's situation was "worse than its ever been".
Ann Clwyd, Mr Blair's human rights ambassador to Iraq, insisted things "overall" in Iraq were improving.
Ms Clwyd said it would be wrong to suggest things were not getting better despite events at Abu Ghraib jail in Baghdad.
"Things are incredibly better now than they were under Saddam Hussein," she told Today.
"In that prison, 2,000 people were killed in one day when Saddam Hussein was in power.
"So let's get it in perspective, these things are awful and they are shaming but the overall picture in Iraq is that things are improving."
Mr Cunningham told Today it was inevitable that the conduct of some American troops had rubbed off on their British counterparts.
"Every right-thinking person is shocked and appalled by the revelation of ill treatment of prisoners in Iraqi prisons by the American military ... as coalition partners some of the blame has been deflected on to Britain and the prime minister," he told BBC Radio 4's Today programme.
Although he acknowledged that the local, European and London mayoral elections would be "difficult" for Labour, Mr Cunningham insisted that he had seen "no evidence of any retreat from support for the prime minister".
"There's a lot of anti-Blair stuff flying around in the press and from his well-known political opponents, some from the left in the Labour party and of course in the opposition parties but that does not equate with any erosion of support for the prime minister in the Parliamentary Labour Party," he said.
He added: "There is no challenge to Mr Blair. This government is firmly in control."
The BBC's Andrew Marr said: "I think he enters a really dangerous period from now until the summer break and it's all about how bad Iraq continues to be."
The Guardian newspaper meanwhile highlighted the fact Chancellor Gordon Brown had held private talks with media tycoon Rupert Murdoch.
Raising the stakes?
It also claimed that supporters of Mr Brown - who is widely seen as having ambitions for the top job - had been "turning up the heat".
On Wednesday Tory leader Michael Howard sought to increase the pressure on Mr Blair with a series of questions about what ministers knew and when of Red Cross allegations of mistreatment of Iraqis by British troops.
Mr Blair said allegations in the February Red Cross report had not been passed to ministers earlier because they were already being probed.
He argued there was no evidence of systematic abuse by British soldiers - saying the Daily Mirror pictures of apparent mistreatment of Iraqis by UK soldiers were "almost certainly fake".
And later, speaking in Downing Street, he said he had become "very angry" on behalf of troops because of all the allegations "swirling around".
The Liberal Democrats will kick off next week with an opposition day debate raising concerns about Iraq and troop numbers.
The party would not support the deployment of additional forces to support the "heave-handed tactics" used by the US in Falluja, said leader Charles Kennedy.
But if British commanders in the Basra sector needed more soldiers to enforce law and order ahead of the 30 June handover of power to an interim government, or to ensure the safety of troops there, "then that request should be considered", he said.