Defence spending: Private Member's Bill calls for 2% UK law
A former Conservative defence minister wants to make it law for the UK to spend at least 2% of its national income on defence.
Sir Gerald Howarth MP will set out the proposal in a Private Member's Bill.
Downing Street said David Cameron would not be allowing government time for it to be debated.
Ministers are under pressure to commit to Nato's target that members should spend at least 2% of gross domestic product (GDP) on defence every year.
Nato figures show the UK is one of just five of the organisation's 28 member states to meet the target this year.
But ministers have refused to say whether the government will do so in future, despite pressure from military chiefs, some Conservative MPs and the United States.
The government has said its plans for future defence spending will be announced in Chancellor George Osborne's autumn spending review.
The new chairman of the Commons Defence Committee, Conservative Julian Lewis, said he was "baffled" by the government's position.
In his first interview since being elected to the role, he said it would be "unconscionable" to drop below 2% of GDP, saying that the "worsening situation" in terms of global security meant that defence spending should be nearer 3%.
Mr Howarth's bill is unlikely to get through Parliament without help from the government, BBC political correspondent Carole Walker says, so it puts the government on the spot.
It will have to decide whether to support a new law with huge implications for future spending or to back away from a Nato target that has been strongly endorsed by the prime minister.
Asked whether Mr Cameron would give his backing to Sir Gerald's backbench legislation, the prime minister's official spokeswoman said: "In terms of the bill, I don't expect it to be given government time."
This means it will go through the usual Private Members' Bill process, which allows a limited amount of parliamentary time to debate such pieces of legislation.
A government source said that the UK's defence spend would include some of a £1bn cross-departmental "conflict pool", in a move approved by Nato.
The Ministry of Defence provides about £450m of the costs of the conflict pool, which is managed with the Foreign Office and the Department for International Development.
Sir Gerald, who had responsibility for the pool when he was in government, said it was not fair to include contributions to the pool in defence expenditure.
"It looks like what the government is doing is raiding a whole series of accounts and saying 'we didn't include those in the defence budget in the past but in order to make up the 2% and satisfy people like me, we are going to raid those accounts'," he told the BBC.