CAIRO, Dec 22 (Reuters) - Egypt's vice president resigned from his post on Saturday, saying he wanted to quit last month but had only stayed on to help President Mohamed Mursi tackle a political crisis.

Mahmoud Mekky, a well-known judge before he took the post, played a leading role in hosting "national unity" talks called by Mursi, although the main opposition politicians stayed away.

Mekky previously said he would step down once a new constitution being put to a vote was approved, as the post of vice president was not outlined in the charter. He had also said he was not privy to Mursi's decree on Nov. 22 to expand his powers that sparked crisis.

In a letter by Mekky that was sent out by the presidency, he said he had worked hard for the nation in his post but that he had "realised for some time that the nature of political work did not suit his professional background as a judge."

He said he had first submitted his resignation on Nov. 7, but it was postponed when Egypt was acting as mediator to try to reach a truce deal in Gaza and then because of other demands, such as helping to organise the "national dialogue".

That dialogue aimed at resolving a row with the opposition over Mursi's extra powers and his decision to fast-track the constitution drafted by an Islamist-dominated assembly to a vote.

In final remarks, Mekky said he had worked hard for the "interests of the nation" and wished continued success to Mursi, who was propelled to power by the Muslim Brotherhood.

Mekky was a leading member of a group of independent judges that started campaigning during the last year's of Hosni Mubarak's rule, when he was jailed for speaking out against election rigging.