French authorities said Thursday that the terrorism alert level in French airports would remain unchanged at "red", the third highest on a four-color scale, after British police thwarted a major terrorist plot.

A French police spokesman said security measures had been stepped up at British airline check-in counters at Paris' main Charles de Gaulle airport.

French airline Air France on Thursday cancelled all flights for London until 1200 GMT.

"All Air France flights (from Paris) to London are cancelled until 1200 GMT due to saturation at Heathrow airport," said a spokesman of the company. There were three flights to be affected.

British police said earlier on Thursday in London that they had thwarted a major terrorist plot to blow up aircraft in mid-flight between Britain and the United States.

British Home Secretary John Reid said the plot was "very significant" and designed to "bring down a number of aircraft through mid-flight explosions, causing a considerable loss of life."

Police said the plot involved hiding explosive devices in hand luggage. A total of 21 suspects had been arrested, a senior police officer said.

Source: Xinhua