Stephen Harper, Canada’s prime minister, will on Wednesday try to persuade US President Barack Obama to exempt Canada from “Buy American” rules – but his country’s own free trade credentials have come under fire.

Mr Harper, who visits Washington on Wednesday, has attempted to corral provinces into opening up to US companies in an attempt to get the administration to change provisions in the $787bn US stimulus package. The US Chamber of Commerce has attacked the US stimulus rules as protectionist and warned of retaliation from other countries.