The amendment gives legal effect to the power-sharing deal signed between President Robert Mugabe and leaders of the two MDC formations, Morgan Tsvangirai and Arthur Mutambara, on September 15 last year.
The amendment creates the new post of prime minister, a position Tsvangirai is set to occupy from next Wednesday.
Under the pact, Mugabe will remain head of state, and the three parties â€” Zanu PF, the MDC-T, and MDC â€”will share cabinet posts.
House of Assembly Speaker Lovemore Moyo said 184 lawmakers in the 210-seat House had voted for the Bill, comfortably surpassing the two-thirds needed, while 72 legislators in the Senate voted for the Bill.
MDC lawmakers shouted â€œtapinda, tapindaâ€ (â€œwe are now in governmentâ€) after the vote count in the House of Assembly.
The passing of the amendment by both houses marks a step towards the constitution of the new government that has been stalled by haggling over posts between Mugabe and Tsvangirai.
MDC-T secretary-general Tendai Biti told the House of Assembly when the Bill was tabled by Justice minister Patrick Chinamasa that it was a â€œmiracleâ€ that the parties had managed to agree on the unity accord.
Biti and Chinamasa are chief negotiators for the MDC-T and Zanu PF respectively.
â€œEverything has happened on the negotiating table other than physical confrontation. It is a miracle that we are here,â€ Biti said. â€œWe go into this government knowing that for this to work there has to be commitmentâ€¦It is important to establish trust from the word go.â€
Chinamasa said negotiations for the establishment of the inclusive government were littered with frustration and disagreements.
â€œIt has been a long, frustrating, quarrelsome journey characterised by animosity and name-calling, but notwithstanding this, what is important is we have reached this path,â€ Chinamasa said.
Fighting between Zanu PF and the MDC-T over the allocation of ministerial portfolios and control of the security forces, appointment of governors, ambassadors and key government officers stalled the deal.
The impasse ended last week after the rival parties agreed to a compromise backed by Sadc.
The MDC-T national council last Friday agreed to join the unity government.
BY BERNARD MPOFU