The Queen nods-off during a lecture on magnets in medicine

She visited Dusseldorf - devastated by allied bombing raids on the night of 3 November, 1944 - on Thursday.

Addressing the Landtag, North Rhine-Westphalia's regional parliament, she said that the suffering of the past could not be forgotten.

During a later engagement, the Queen appeared to fall asleep during a talk on the use of magnets in healthcare.

She slumped in her chair and closed her eyes for about 10 seconds during the lecture at the Heinrich-Heine University clinic.

'Achieved together'

The Queen, 78, also made an uncharacteristic error during her address to the Landtag.

She accidentally skipped a paragraph before stuttering and returning to the correct place.

The Queen with Peer Steinbrueck, prime minister of the Landtag

During her address she said: "I depart with renewed confidence in the deep friendship between our two countries.

"I know that many of you in Dusseldorf and elsewhere in Germany have had recent cause to remember.

"But we have travelled a very long way in the last 60 years and so much of what has been achieved has been done together."

The Queen told the Landtag that mutual Anglo-German investment, scientific exchange and commercial co-operation was now ingrained but should not be taken for granted.

She said: "I leave Germany this afternoon at the end of this visit with a real sense of opportunities ahead for those in both our countries who can grasp them."

During her visit, the Queen visited the grave of a forgotten World War I hero Major Charles Yate in Stahnsdorf.

She also met former Manchester City goalkeeper Bert Trautmann OBE, a former German POW who played for Manchester City in the 1956 FA Cup Final against Birmingham, despite breaking his neck.