British special forces have reportedly been operating on the frontline in Syria.
Elite UK soldiers crossed from bases in Jordan to help a unit of the New Syrian Army (NSA) rebel group being attacked by Islamic State, according to the Times.
It comes after reports that UK special forces units were on the ground in Libya, where they blew up an IS suicide truck in May.
MPs voted against British military action in Syria in 2013, but the use of special forces does not require the approval of Parliament.
NSA First Lieutenant Mahmoud al-Saleh told the Times that British troops had provided logistical help to rebuild the defences at a base in a village called al-Tanf, near the borders with Jordan and Iraq - including after a suicide attack which killed 11 rebels.
He told the paper: "They helped us with logistics, like building defences to make the bunkers safe.
"They (IS) attack us at all times, 3am, 5am, 4pm, 11pm. If you look at the timing of the assaults it's clear they don't want us to get any rest. They're using missiles, mortars and many suicide bombers,"
A Ministry of Defence spokeswoman said they do not comment on special forces operations.
In December, MPs vote by 397 to 223 - a majority of 174 - to back David Cameron's plan to extend airstrikes against Islamic State from Iraq into Syria.
But that came more than two years after the Prime Minister was defeated in his attempt to gain support for military intervention, which has now endured five years of conflict that has killed hundreds of thousands and sparked the refugee crisis spreading from the Middle East into Europe.
Mr Cameron was defeated by 13 votes, the first such reverse on foreign policy for at least 150 years. He immediately ruled out action, and a proposed US-led intervention against the regime of Bashar Assad was abandoned soon after.
Crispin Blunt, who chairs the Commons foreign affairs select committee, said he would be "not particularly concerned and not at all surprised".
"This kind of operation has been, to a degree, briefed out by His Majesty King Abdullah of Jordan. it is part of a series of operations in support of the Jordanians and the Jordanians supporting elements of the Free Syrian Army," he told BBC Radio 4's Today.
"This seems to be a small set of operatives who deserted from Assad's army some time ago and got trained up by the Americans and ourselves."
Asked about Parliamentary approval, he said: "We are in this Alice in Wonderland world where Parliament has approved a motion saying it 'notes the Government will not deploy UK troops in ground combat operations'.
"It doesn't say in brackets 'not special forces', but the convention is that it is .. because we don't comment on special forces operations.
"If you run an operation for a long time, as we have here and in Libya, eventually newspapers like The Times report it and then the Defence Secretary can't talk about it and we can't have a proper conversation about how it fits in a wider UK strategy."
Mr Blunt said the country still lacked "a proper UK role within a proper international strategy that is - together with the Russians and other people effective on the ground - about defeating Isil and al Nusra".