The UK has failed to act on promises to plug loopholes that allow the sale of arms to countries with poor human rights records, aid agency Oxfam says.
It says that military vehicles were sold to Uganda by a South African subsidiary of the UK firm BAE Systems.
These were used to quell demonstrations and disperse opposition supporters as recently as 10 days ago, it says.
President Yoweri Museveni won polls last week amid criticism by EU observers that his campaign was unfair.
The British High Commission in Uganda say they are studying the report carefully and take seriously any allegations of human rights abuses.
In the report, Oxfam accuses the UK government of failing to act on its promises to secure an international arms trade treaty which would close loopholes in the law relating to the sale of weapons to countries with poor human rights records.
It reveals how a South African subsidiary of the British company BAE Systems sold Mamba armoured personnel carriers to the Ugandan government ahead of last week's elections.
The report claims at least 32 such vehicles have been sold by the subsidiary, called Land Systems OMC, since 2002, with the most recent consignment arriving just ahead of polling day.
The particular concern that Oxfam identifies is that the lack of international controls means that an overseas subsidiary can secure sales in military equipment which a UK-based company would not be allowed to do.
Oxfam alleges that the vehicles supplied to Uganda were used to quash demonstrations in Kampala following the arrest of opposition leader Dr Kizza Besigye at the end of last year.
They also claim they were used during an opposition rally a week before the polls in which three people died.
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