In a revelation that will put pressure on Jack Straw, the foreign secretary, to give a full account of what he knew and when, a legal statement by a former South African intelligence officer claims he warned MI6 and the Pentagon of an attempt to overthrow the president of Equatorial Guinea.

The officer, Johann Smith, said he circulated two briefings in December 2003 and January this year to “Michael Westphal of the US (a Pentagon official) and British SIS (Secret Intelligence Service — MI6) contacts”. One memo accurately predicted the coup was due to take place in March.

Simon Mann, a former British special forces soldier, and 67 mercenaries were arrested on March 7 in Zimbabwe as they were arming themselves with machineguns, mortars and rocket-launchers to mount the attack. They apparently planned to oust President Teodoro Obiang Nguema and install an exiled politician instead.

A senior aide to Straw said on Friday that the Foreign Office no longer stood by statements given to The Sunday Times in August that the government had no advance knowledge of the plot. The official admitted the paper had been given incorrect information.

In August, the paper wrote that MI6 and the Pentagon had been made aware of the plot “through diplomatic channels”. This was denied at the time by the government.

Straw later admitted in a parliamentary answer that the government did learn of the coup through a “confidential diplomatic exchange” two months before it happened.

This weekend, a spokesman for Michael Ancram, the shadow foreign secretary, said Straw should tell MPs exactly what the government knew and when.

Ancram’s move follows Straw’s admission in a parliamentary answer to him earlier this month that the government did have prior warning. The Foreign Office claimed it had been aware of rumours about a coup because of reports in the Spanish media in January and another source of information.

Straw refused to identify the source, saying the information was contained in a “confidential diplomatic exchange”.

He added that officials could find no definitive evidence of the plot and therefore did not warn the Obiang government.

He said: “We took action to try to establish whether any UK companies were involved and to underline our opposition to involvement by any UK company in such activities.”

The Foreign Office carried out an investigation, but apparently failed to establish that Mann’s company, Logo Logistics, was registered in Guernsey. Several more Britons were also allegedly involved in organising and financing the coup but the Foreign Office appears to have done little to investigate this.

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