"We have seen no evidence of systemic manipulation of the process. But there are disturbing indications that the collation process -- where the votes are finally counted -- may be subject to deliberate political interference," the two diplomats said in a joint statement.
Britain and the United States welcomed "the largely peaceful vote" after it was pushed into a second day due to failures in new technology designed to read biometric identity cards. The cards were introduced to combat electoral fraud.
They added that their governments "would be very concerned by any attempts to undermine the independence" of the electoral commission.
Nigeria's presidential election pitting President Goodluck Jonathan against former military ruler Muhammadu Buhari is the closest in Nigeria's history, and first to present a credible opposition challenge.