British Prime Minister David Cameron declared security his main concern on the eve of the London Olympics Thursday while US presidential hopeful Mitt Romney backtracked on unflattering comments he made.

"The biggest concern has always got to be a safe and secure Games," Cameron told a press conference with the Olympic Stadium as a backdrop. "That matters more than anything else.

"I think we've made as many preparations as we can. I think we have very good contingency plans in place.

"Always as prime minister I feel that is an area I should take personal responsibility for."

London's security preparations had been called into question in comments by Republican candidate Romney, who is in London to attend the opening ceremony on Friday.

Romney said several issues in the build-up to the Games had been "disconcerting", focusing on a fiasco involving private security contractor G4S which has failed to provide the number of private guards it had promised.

"The stories about the private security firm not having enough people, the supposed strike of the immigration and customs officials -- that obviously is not something which is encouraging," he told NBC News on Wednesday.

It was "hard to know just how well it will turn out", Romney added.

But Romney, who took over the preparations for the 2002 Winter Games in Salt Lake City after they were marred by scandal, struck a more diplomatic note after he met British opposition Labour Party leader Ed Miliband.

"It is impossible for absolutely no mistakes to occur," Romney said.

"Of course there will be errors from time to time, but those are all overshadowed by the extraordinary demonstrations of courage, character and determination by the athletes.

"The Games are, after all, about the athletes, the volunteers and the people of the community who come together to celebrate those athletes. As soon as the Games begin, we all forget the organisers and focus on the athletes."

Britain has deployed an additional 4,700 troops in recent days to make up the shortfall in private security guards, taking the total military deployment to 18,200.

The huge operation to safeguard the Games is Britain's biggest peacetime security mobilisation.

Meanwhile, Games organisers and Cameron tried to downplay a mix-up which saw North Korea's women football team walk off the pitch in an embarrassing start to the competitive action at the Games.

The International Olympic Committee (IOC) apologised to the North Korean delegation after its football squad were shown on giant screens next to South Korea's flag, at their opener against Colombia on Wednesday at Hampden Park in Glasgow.

Relations between the two Koreas, still officially at war and facing each other over the world's most heavily guarded border, have worsened in recent months over the communist North's nuclear programme.

The North Korean players left the pitch in protest but were persuaded to return an hour later after the mistake had been rectified, and went on to record a 2-0 victory.

"This was an honest mistake, honestly made," Cameron said.

"An apology has been made and I'm sure every step will be taken to make sure these things don't happen again.

"We shouldn't inflate this episode. It was unfortunate, it shouldn't have happened, and I think we can leave it at that."

IOC president Jacques Rogge described the incident as 'most unfortunate'.

"I can assure you the organising committee will take corrective action and there will be no repeat," he said on Thursday.

In a lighter comment, Cameron said he would watch the judo events at the Games with Russian President Vladimir Putin -- but joked it could get in the way of their trade talks.

"I discussed EU bilateral trade with the Brazilian president (Dilma Rousseff) yesterday -- she's here for the Games," he told business leaders in London.

"I will do the same with the Russian president next week when he's here," he added.

"We will be at the judo, so it may be a bit off-putting."