BBC Reports:

Mr Brown is on his way to officially tender his resignation to the Queen, and recommend that Conservative leader David Cameron should succeed him.
Speaking alongside his wife Sarah outside No 10 Downing Street, he said the job had been “a privilege” and wished his successor well.
His decision comes as the Tories and Liberal Democrats are poised to agree a deal to form a government.
[. . .]
“I wish the next prime minister well as he makes the important choices for the future,” he said.
His two young sons joined him and wife Sarah for his brief statement which ended with the words: “Thank you and goodbye.”

UPDATE: Via the Guardian, the British Press Association reports Brown has told told friends he plans to resign from parliament entirely and leave politics.