Google's tax deal with the Treasury could be investigated if something appears "not as it should be," the European Union's Competition Commissioner has said.
Margrethe Vestager told BBC Radio 4's Today programme she is able to "use state aid tools to look for more individual companies and more selective advantages being given out in the form of tax rulings."
Her comments come a day after David Cameron was forced to defend the Treasury's tax deal with Google that sees the internet giant pay £130 million on profit estimated at £6 billion backdating 10 years.
Danish politician Ms Vestager described so-called "sweetheart deals" for big companies as "unfair" and sometimes "illegal state aid."
Asked whether she would examine the situation, the commissioner said: "That is way too early to say because I don't know the details of the deal.
"If we find there is something to be concerned about, if someone writes to us and says this is maybe not as it should be, then we will take a look."
Later on the programme, Labour MP Margaret Hodge, former chair of the Commons Public Accounts Committee, said Ms Vestager should "take a look" at the deal.
"The whole thing here is that we need greater transparency. If we think it is unfair we lose confidence in the integrity of the tax system."
Google should pay "much more" tax in the UK, one of the company's biggest British shareholders has said.
James Anderson - whose Scottish Mortgage Investment Trust owns £120 million of shares in Google's parent company, Alphabet - said it was in the company's own interest to pay a "decent" rate of tax.
His comments in The Times come after David Cameron was forced to defend the Treasury's tax deal with Google that sees the internet giant pay £130m on profit estimated at £6 billion.
"My take remains that it is in the long-term interests of Google and others of that ilk to pay decent rates of tax and that they and others would be best served in taking the lead in volunteering," said Mr Anderson.
"They are beneficiaries of state spending at many levels and in return they would get respect."
ITV News Political Editor Robert Peston tweets:
Google will appear at Public Accounts Committee on tax, I have learned
The government has been criticised for a deal that lets internet giant Google pay £130 million tax on £6 billion profit - a rate of around 3%.
But do you think big corporations should pay more tax? Take our poll and have your say.
The prime minister responded to Labour's questions about Google's "sweetheart" tax deal by blaming the previous Labour government for failing to collect funds from the internet giant.
"We're taking about tax that should have been collected under a Labour government," David Cameron told Labour leader Jeremy Corbyn at today's Prime Minister's Questions.
Watch David Cameron's passionate defence of tax collection under the Tories:
I'm absolutely clear that no government has done more than this one to crack down on tax evasion.
...We've put in place the diverted profit tax that means this company and other companies will pay more in future and more than they ever paid under Labour, where the tax rate for Google was 0%.
Mr Cameron also said that the Conservative party has raised an extra £100 billion from businesses by changing tax laws.
Mr Corbyn accused the prime minister of failing to answer his question - whether he disputes that Google is paying around 3% tax in the UK.
A former senior strategy adviser to David Cameron has said that there is a sense big firms appear to operate "above the law" as the Treasury faces further questions over its tax deal with Google.
Steve Hilton, who worked for the prime minister until 2012, told BBC Radio 4's Today:
There is a growing sense that companies that are so big and so dominant - not just in the marketplace but in the way they relate to governments, their lobbying efforts and so on - that they really are above the law.
In this particular case they have made clear that they were abiding by the law then, when the arrangements caused anger, and now that they have new arrangements.
...We have really got to make clear to businesses that they have a responsibility to behave in a way that earns public trust.
George Osborne has been criticised for a deal that lets the internet giant pay £130 million tax on £6 billion profit - a rate of around 3%.Read the full story ›
Google is not the only big firm to come under scrutiny over low UK tax contributions.Read the full story ›
Shadow chancellor calls for the National Audit Office to launch an inquiry into "derisory" tax dealRead the full story ›
Treasury minister David Gauke told MPs that while the issue would be raised, the UK remained constrained by European rules.Read the full story ›