South African police have launched an investigation after a fan breached security and burst into England's locker room in Cape Town, minutes after a visit by Princes William and Harry.

"It is totally unacceptable for FIFA that a fan can find his way to the dressing room of any team in any FIFA competition," FIFA spokesman Nicolas Maingot told journalists.

"Security will be tight to ensure it will never happen again," he added.

But David Beckham and the two royals brushed off the incident, which came just after the team was booed off the field for their 0-0 draw with Algeria late Friday.

"The actual fan literally just walked in very casually and just said something to me and then walked out - there was no scuffle, there was no aggression at all," he said at a Football Association reception in Johannesburg.

"He didn't comment on the performance. He literally walked in, said hello."

"Luckily it was after the princes had left - five to 10 minutes after. Obviously it's been blown out of all proportion as well," Beckham said.

"Harry and I left the door open, that's why it happened, it was our fault," William joked.

South African police said a member of FIFA had escorted the man out of the locker room, and that security guards were being interrogated about why he was let go.

No one was injured and nothing was reported stolen but the English Football Association has made an official complaint to FIFA.

Security has been a chief concern for organisers ever since South Africa was named the World Cup host six years ago, with worries focusing mainly on the country's high crime rate.

But stadium security has moved to the fore as stewards at Cape Town and three other venues walked off the job last week in a pay dispute that forced police to take over security around the fields.

Protesting stewards twice clashed with police over the last week, once in Durban and once in Cape Town, as they tried to demand their salaries from Stallion Security, the firm that had been contracted by FIFA to screen fans entering the gates.

Police were also under pressure to crack down on illegal ticket sales on Saturday, with Johannesburg's Saturday Star newspaper proclaiming "Ticket crooks rule" across its front page.

South Africa outlawed the resale of tickets just before the tournament started, meaning tickets can only be legally transferred through FIFA.

Scalpers can face a 15,000-rand ($A2301) fine and up to five years in prison. A Nigerian man was slapped with a three-year sentence this week after he was found with 30 tickets that could not be verified.

Two men had their tickets for Friday's US-Slovenia match confiscated just before the game as they were apparently trying to sell them outside the stadium, police said.

But one officer at the gate to Ellis Park stadium in Johannesburg said police struggled to keep out scalpers, as fans desperate for tickets show up at the stadiums hoping to find a way in.

"We are trying to keep them away because it is illegal," he told South Africa's Sapa news agency. "But they are moving around all the time."

FIFA has cumbersome rules for reselling tickets, which can only be done online and not within three days of the match.

The world football body already drew flack for its online ticket sales system in a country where Internet access remains a luxury.

The threat of stiff penalties has not deterred scalpers from openly circulating around stadiums, and their numbers appear to be growing as the group stages advance and games become more critical to teams' hopes of making the next round.

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