The shadow chancellor is facing criticism from his own side over the Tories' failure to match Labour defence and international aid cash.

Oliver Letwin was forced onto the back foot on Sunday as he sought to attack the upcoming government comprehensive spending review.

He promised that "frontline" military spending would not be hit under a Conservative administration after shadow Cabinet colleague Nicholas Soames reportedly slammed as "outrageous" plans for a £1.5 billion reduction in the Ministry of Defence budget.

"We are absolutely guaranteeing that we will spend what it takes to maintain the fighting capacity of our defence forces," Letwin told the BBC's Breakfast with Frost programme.

"We have at the moment 80,000 to 90,000 officials in the Ministry of Defence.

"And all over Whitehall we have vast numbers of people who are doing things which are not the frontline and that is what we are need to thin away.

"We have a problem of fat government and when we thin it down by not doing so much governing, not interfering so much in people's lives, we can deliver more money to the frontline without raising taxes and, indeed, while reducing them."

Savings

Letwin has promised to protect health, education and pensions budgets but wants to find savings elsewhere in a bid to reduce the state's share of national spending.

However, like Soames, shadow international development secretary John Bercow is putting pressure on his colleague to equal the Treasury's commitment to overseas aid.

Some shadow Cabinet members fear being outflanked by Labour and the Liberal Democrats in the general election campaign through attacks on a Tory cuts agenda.

"I'd certainly like to match Labour's target," Bercow told GMTV on Sunday.

"It isn't for me to decide, it's for Oliver Letwin, the shadow chancellor to make a judgment.

"But if you're asking me my opinion... that there needs to be a demonstrable financial commitment by the Conservative Party to helping the poorest and most vulnerable people on the planet seems to me to be so obvious as to be beyond argument."

And Bercow insisted the issue was not closed.

"He [Letwin] certainly hasn't said to me, John, I aim to slash your budget.

"He has indicated he recognises the importance of the commitment by the Conservative Party in terms of development assistance as it's officially known, I want that to be a substantial commitment, I am making the argument and happy to justify it."