Britain said on Thursday that the re-election of Zimbabwe's President Robert Mugabe could not be deemed credible without an independent investigation into allegations of voting irregularities.

The comments from Zimbabwe's former colonial ruler came on the day that Mugabe, Africa's oldest and one of its longest-serving leaders, was inaugurated for a new five-year term.

"I strongly believe that an independent investigation of any allegations of election irregularities would be required for the election result to be deemed credible," British Foreign Secretary William Hague said in a statement.

Last week, the main opposition party in Zimbabwe, the Movement for Democratic Change (MDC), dropped a court challenge against the election result because of doubts it would receive a fair hearing.

Hague said he was "extremely concerned" at the decision to abandon that challenge, saying he was disappointed that the South African Development Community had chosen to endorse the election result.

Mugabe, 89, last week told critics of his re-election to "go hang", making clear he would brook no questioning of his disputed victory, either from the West or from his MDC rival.