The Daily Telegraph suggests the UK's international development spending, which is set at 0.7% of GDP, could be higher by 2030.
It's understood backbench MPs are looking to force the government to commit 2% of the nation's income to defence. A vote could take place as early as next week.
Speaking to the Telegraph, Conservative MP John Baron said: "The fact that DFID spending could one day match defence spending illustrates the folly of ring-fencing departmental budgets.
"We should never forget that the first duty of Government remains the defence of the realm.
"In a world where many countries not necessarily friendly to the west are re-arming and becoming more assertive, we should instead be spending more on our Armed Forces."
Christian Aid claims the article is "irresponsible and seems to be based on incredibly weak assumptions".
Head of Advocacy, Laura Taylor, said: "Aid has increased quite rapidly over recent years in order to meet the long-standing promise to provide just 0.7% of gross national income as aid to the world's poorest people.
"However, now that this target has been met, aid will only continue to increase if the economy grows overall, and in all likelihood is likely to plateau.
"Likewise, just because defence spending has decreased slightly in recent years, it seems far-fetched to assume that this reduction will continue year-on-year. Giving aid is an extremely practical way of demonstrating our love for our neighbours in need and UK aid is helping transform many lives around the world."
Gavin Crowden, Head of Policy and Public Affairs at World Vision UK, said: "It is vital that taxpayers' money is spent in the right way, in the right places, and on the right programmes. And international aid is no exception.
"For children living in the poorest communities in the world, I am proud that UK aid supports and enables them. The UK aid policy should not be judged by how much is spent, but by what it delivers.
"You need only look at the conflicts in Syria and South Sudan to see why UK aid is so important. Political leaders and governments are struggling to agree on effective solutions yet aid agencies are reaching those most in need with food, shelter and medical care."