The US Transportation Department has opened an investigation into brake problems in the 2010 Toyota Prius.
The move follows an admission from Toyota that it had had a problem with the brake system in the hybrid, which the carmaker said was fixed in January.
The National Highway Traffic Safety Administration has received 124 reports from drivers about the issue, including four of crashes.
There have been no reports of any such accidents in the UK.
The investigation will look into allegations of momentary loss of braking capability while travelling over uneven road surfaces, potholes or bumps. However it will not suspend sales.
This latest alarm for the beleaguered carmaker - the world's number one - follows worldwide recalls of almost eight million cars due to separate floor mat and pedal problems.
No Prius recall
Darren Brown, Prius driver, Belfast
"I was frankly relieved when I heard that problems with the Prius brakes were being reported because my wife and I thought we were the only ones.
We bought one of the first Prius cars [of the third generation model] in Northern Ireland in July and within about two weeks we noticed its brakes weren't performing consistently.
Basically if you are slowing down with the brake pedal pressed coming up to a junction or traffic lights, if you hit a pot hole with one of the wheels the brakes completely release for a couple of seconds. It is enough to send you freely rolling forward about four to six feet before they re-apply.
The whole thing is very unnerving but now others are reporting problems it is good to know we're not alone."
Toyota UK told the BBC it was aware some users had reported problems but investigations had revealed no faults. Concerns have been put down to drivers not being used to the particular way the brakes feel in the Prius.
Toyota vehicles made and sold in South Africa are the latest to be recalled, a company spokesman has told the BBC.
The Japanese government has also ordered Toyota to investigate brake problems but the company says it was "alleviated" at the end of last month by making changes to the software in the braking system.
Toyota's managing officer, Hiroyuki Yokoyama, said although the company had found a clash between the anti-lock brake system (ABS) and regenerative braking, more investigation needed to be done before deciding on whether to issue a recall on the Prius.
Earlier, he told a press conference in Japan: "As for whether this will mean a recall, we are currently looking into what we can do as soon as possible for our customers to buy our vehicles. However, we hope for a bit more time before deciding on specific measures."
As depressing the brakes further activated normal braking, Toyota said the glitch was not legally a safety hazard and said it had received no reports of any accidents related to it.
Mr Yokoyama said Toyota was listening and responding to its customers' concerns: "When we've been told something by our customers, our goal is to respond as soon as possible, and we have already changed the design for the Prius from January."
Toyota's admission follows 200 reports of complaints from drivers in the US and Japan. There have been no braking problems reported in Europe, the carmaker said.
In the UK, it confirmed that it would be nearly a week before it could start repairs on cars with defective accelerator pedals.
Earlier, Toyota reported a huge swing back into profit in the last quarter of 2009.
Its net income was 153 billion yen ($1.68bn; £1.06bn) after a loss of 164 billion yen a year earlier.
Toyota confirmed its estimate that it would lose about $2bn (£1.23bn) in costs and lost sales from its worldwide recall of potentially faulty vehicles.
It added it had not yet worked out the cost of the latest reports of brake problems with the new Prius.
However, the firm said it still expected higher sales and to make a profit this year, despite the heavy blow to the company's reputation.
TOYOTA RECALLS: STORY SO FAR
September 2007, US: 55,000 Camry and Lexus cars in floormat recall
October 2009, US: 3.8m Toyota and Lexus vehicles recalled due to floormat problem
November 2009, US: floormat recall increased to 4.2m vehicles
January 2010, US: 2.3m Toyota vehicles recalled due to accelerator pedal problems (Of those, 2.1m already involved in floormat recall)
January 2010, US: 1.1m Toyotas in floormat recall
February 2010, Europe: 1.8m Toyota's in pedal recall
February 2010, Japan, US: 200 reports of brake faults in new Prius. None recalled
Shares in Toyota hit their lowest level for 10 months on the Tokyo Stock Exchange on Thursday, with continuing concern about the safety of the company's vehicles.
Its eight million recall total includes 1.8 million cars across Europe - 180,865 of those in the UK.
The seven models being recalled in Europe are the Aygo, iQ, Yaris, Auris, Corolla, Verso, and Avensis, and cover manufacturing dates going back to February 2005.
In the US, they are the RAV4, Corollas, Matrix, Avalons, Camrys, Highlander, Tundra, and Sequoia, and cover dates going back to October 2005.
The parts needed to repair the cars will not arrive in the UK until next week, with the first repairs scheduled for Wednesday. Toyota says the process, which should only be carried out by its dealers, takes about half an hour.
The carmaker said it was not aware of any accidents resulting from the issue and that only 26 incidents involving accelerator pedals had been reported in Europe.
In a separate development, Ford said on Thursday it would repair up to 17,600 of its Ford Fusion and Mercury Milan hybrids built before mid-October that could suffer braking problems
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