A British company hired by the US military to train Afghan intelligence officers spent $50million on high-end cars and paying the bosses' wives six-figure salaries.

According to a Pentagon audit, which was made public on Wednesday, New Century Consulting, blew $42,000 on automatic weapons.

The firm billed the US government for Porsches and an Aston Martin and also paid the 'significant others' of the firm's top executives six-figure salaries - despite there not being proof they did any work.

New Century Consulting - founded by Colonel Tim Collins OBE - provides 'bespoke and cost-effective' military skills and specialist police training around the world.

Sen. Claire McCaskill (pictured) said New Century Consulting showered other personnel with hefty pay and bonuses they hadn't earned

In 2015, it was under criminal investigation and faced questions over a £116million US contract to train Afghan security forces in counterinsurgency. The company denied any wrongdoing.

And on Wednesday, Sen. Claire McCaskill, said New Century Consulting showered personnel with hefty pay and bonuses that they hadn't earned.

Overall, the military contractor 'left taxpayers on the hook for over $50million in questionable costs,' McCaskill said in a statement.

McCaskill, the top Democrat on the Homeland Security and Governmental Affairs Committee, summarised the audit's major findings in a letter to Defense Secretary Jim Mattis.

She demanded to know which Defense Department office was responsible for overseeing the contractor, what steps are being taken to recover the disputed payments, and whether New Century Consulting will face disciplinary action.

Michael Grunberg, chief executive officer of New Century Consulting, said the company is being portrayed unfairly and that it strives to follow federal acquisition rules.

Grunberg said it 'is most unfair and is significantly inaccurate' that the executive assistants received excessive salaries.

He said the audit 'questioned solely the use and depreciation treatment of vehicles' and that New Century Consulting 'accounted for no more than three vehicles across the entire business at any one time.'

The purchase of the weapons was done properly and at the direction of the US-led command overseeing the training and equipping of the Afghan security forces, according to Grunberg.

McCaskill's disclosure of the audit's key findings is a rare glimpse into the opaque world of battlefield contracting.

Contractors are indispensable in Afghanistan, handling security, transportation, construction and more.

Yet the Defense Department has faced widespread criticism that it often fails to perform rigorous oversight of the companies and how exactly US taxpayer dollars are spent.

The Defense Contract Audit Agency examined New Century Consulting's invoices between fiscal years 2008 and 2013, when it was a subcontractor to another company, Imperatis Corporation.

Among the costs charged to the US were expenditures for seven high-end cars - Porsches, Alfa Romeos, a Bentley, an Aston Martin and a Land Rover, according to McCaskill's letter to Mattis. The actual cost of the vehicles isn't specified.

New Century Consulting - founded by Colonel Tim Collins OBE (pictured) - provides 'bespoke and cost-effective' military skills and specialist police training around the world

McCaskill told Mattis: 'NCC claimed that the vehicles were available to all employees but the vehicles actually were used exclusively by the chief executive officer, chief operating officer, chief financial officer and the significant others of the CEO and CFO.'

Her letter doesn't identify who the significant others are.

These 'significant others' also were employed by New Century Consulting as executive assistants and had an average salary in 2012 of close to $420,000 each even though McCaskill said the company was unable to provide evidence they actually performed any work.

The audit also challenged millions of dollars in compensation for other employees, including the consultants whom the company sent to Afghanistan to train the forces there.

McCaskill said the consultants were supposed to be paid at a 100 per cent rate when deployed overseas, but only at 60 percent when on leave.

But New Century Consulting gave its consultants the full rate regardless of where they were.

She wrote: 'These excessive payments cost taxpayers over $15 million.'

New Century Consulting also gave its consultants more than $3.3 million in bonuses that they either didn't earn or that weren't required by their contracts, according to the senator.

Army Contracting Command didn't respond to a request for comment.

The report also comes amid the tense debate inside the Trump administration over the way ahead in Afghanistan.

Two of President Donald Trump's most senior advisers - chief strategist Steve Bannon and son-in-law Jared Kushner - have been advocating for military contractors to fight the war there instead of American forces.

The United States has about 8,400 troops in Afghanistan, and so far Trump has resisted the Pentagon's recommendations to send as many as 4,000 more.