Lord Robertson, the Labour peer, is under pressure to step down as a director of a Scottish company that has business dealings in Zimbabwe.

The former defence secretary and Nato secretary-general is paid £39,000 a year as a non-executive director of the Glasgow-based Weir Group.

In March the firm paid £113m for CH Warman Pump Group (CHW), a specialist manufacturer with bases in Zimbabwe and other African countries.

Robertson’s involvement in a company which continues to trade under Robert Mugabe’s despotic regime has put him at odds with Labour policy.

Gordon Brown has urged British companies not to invest in Zimbabwe. Yesterday the prime minister signalled his intention to press ahead with tougher sanctions against the country, despite the embarrassing veto of his plan for a UN arms embargo and travel ban.

Officials were hastily drawing up a “plan B” after Russia and China ambushed Britain at the UN security council in New York.

No 10 now hopes the European Union and the United States can draw up a joint agreement that would include a ban on trips abroad for any Zimbabwean linked to Mugabe’s regime and new efforts to freeze the assets of the Zanu-PF leadership.

Yesterday, David Miliband, the foreign secretary, could not hide his disappointment at the collapse of the government’s Zimbabwe policy. He, insisted Britain was right to push the issue to a vote despite the threat of a Russian and Chinese veto.

“The UN has been saying for long time this is a real problem,” he said. “So we said ‘Let’s have real action’. It is right that in the end people show their cards. You have to get people to front up. There was hiding going on.”

Weir acquired CHW, which has an annual turnover of £40m and employs 430 people, last March. As part of the buyout , it acquired 100% of the share capital of Warman (Zimbabwe), which provides sales, distribution and after sales services for equipment used in platinum mining and has an estimated annual turnover of £500,000.

Weir’s chief executive Mark Selway has said he believes opportunities in Zimbabwe would be “quite significant” in the medium term.

Ed Davey, the Lib Dem foreign affairs spokesman, said: “With Gordon Brown making such tough noises on Zimbabwe, it would be grossly hypocritical if a Labour peer had not ensured that the company on whose board he serves is not upholding both the spirit and the letter of government policy.

“Companies like Weir need to look closely at whether their investments assist Mugabe’s regime in any way, whether through providing much-needed foreign exchange or direct revenue to the government.

Angus Robertson, the SNP’s foreign affairs spokesman, said: “If George \ is fully aware and content with this commercial decision it is definitely at variance with the views of the prime minister and that is not a tenable situation for him to be in.

“Now is a time for all people of influence to do their utmost to support democracy in Zimbabwe and not prop up the Zanu-PF regime directly or indirectly.”

However, Lord Robertson defended the company’s deal, saying: “Weir inherited Warman’s small office in Bulawayo which has insignificant business of £500,000 a year and the group has not invested in Zimbabwe since the acquisition. In no way could this give comfort to Robert Mugabe.”

A Weir spokeswoman refused to comment on the ethics of investing in Zimbabwe, adding: “We wouldn’t get involved in politics. We follow our global customers. A number of them operate in the mining market generally in Africa and some of them operate in Zimbabwe. We are supplying to a number of global operators.”

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