London wheat fell back below £100 a tonne on Thursday after upbeat comments from Bank of England policymaker Paul Fisher sent sterling soaring.

Mr Fisher said that the Bank's controversial quantitative easing programme, which has involved pumping billions of pounds into debt markets to encourage lending, was "having the scale and speed of impact that we would have hoped for when we started in March".

The show of faith in a policy whose effectiveness some economists have questioned sent sterling up more than 2% against both the dollar and the euro, against which it recovered to E1.09, and by nearly 3% against the yen.

This rise, in turn, undercut the competitiveness of UK exports, including grain, and quashed a rally in wheat which had taken prices up more than 15% between a low on September 9 and Tuesday's high.

"After a good run up over the past few days, a level of profit taking has also been seen," Hugh Schryver, at Glencore, said.

 London wheat for November tumbled 3.4% to £99.70 a tonne at one stage before recovering some ground to close at £101.00 a tonne, down £2.25 on the day.