Gordon Brown has broken the official code governing cabinet ministers' behaviour and failed to comply with Government rules on overseas travel.
The Chancellor was last night accused of trying to cover up his breach of the code by giving evasive answers to questions in Parliament.
Mr Brown failed to gain written permission from the Prime Minister to travel to Nigeria in May 2006. The Treasury later said that arrangements for the trip were "in line with normal procedures", but a Freedom of Information request has revealed that the Chancellor did not gain the Prime Minister's written permission to travel abroad.
The ministerial code states that cabinet ministers travelling outside the EU on official business must "seek the Prime Minister's written approval".
Conservative MP Chris Grayling said: "Relations between Gordon Brown and Tony Blair have deteriorated so much that he'd rather break the ministerial code than formally notify his neighbour about his overseas travel. This is yet further evidence of the paralysis that's gripping this government."
A spokesman for Mr Brown said that No 10 was "fully informed about this before the trip took place. No 10, which is responsible for the ministerial code, is satisfied that all the requirements were met. You would think that the Tories would get behind the campaign for free education in Africa, which this trip was about, instead of this ridiculous quibbling about Whitehall bureaucracy."Reuse content