Cameron held talks in the South African capital Pretoria with President Jacob Zuma on the first leg of a trip which will also take him to the continent’s other major power, Nigeria.

But while the two leaders tried to bridge their gap over the Libya conflict and build up trade, their joint press conference was dominated by the hacking crisis which claimed its most senior victim while Cameron was on his way here.

Cameron was told about the resignation of Paul Stephenson, the Commissioner of the Metropolitan Police, just over an hour after taking off from London on what is his first visit to sub-Saharan Africa since coming to power last year.

Asked if he was making a mistake by being in Africa at such a crucial time, Cameron was forced to defend his visit and called for an extension of parliament which is currently due to break up for the summer on Tuesday.

“I am asking for parliament to sit an extra day on Wednesday so I can make a new statement adding to the details of the judicial inquiry,” he said.

Stephenson quit amid pressure over his ties to the scandal-hit News of the World, but he issued a parting shot at the premier’s own decision to employ an ex-editor of the tabloid as his media chief.

“The situation in the Metropolitan Police Service is really quite different to the situation in the government, not least because the issues the MPS is looking at are the issues… that have a direct bearing on public confidence into the police inquiry into News of the World, and indeed into the police themselves,” Cameron said.

The prime minister has already shortened the visit which originally was due to last five days and take in the new state of South Sudan as well as Rwanda.

“It is right for Britain to be engaged with South Africa and to be engaged with Africa as a whole. There is a huge opportunity for trade, for growth, for jobs,” Cameron said.

Cameron said he and Zuma reaffirmed commitment to double bilateral trade by 2015, and repeated his support for a pan-African free trade area currently under negotiation.

He argued that a free trade area could increase gross domestic product across the continent by an estimated $62 billion a year — $20 million more than the world gives sub-Saharan Africa in aid.

Cameron and Zuma also discussed the conflict in Libya, where South Africa has accused NATO of overstepping its UN mandate to impose a no-fly zone.

Zuma again refused to back Britain’s call for Libyan leader Moamer Kadhafi to step down, saying his future should be decided in talks among the warring parties.

“What happens to Kadhafi must be decided by the Libyan people. You need to negotiate how, why and where he must go,” Zuma said.