Swansea West MP Geraint Davies said it was vital for a "greener future", and accused ministers of "back-pedalling" on their commitment to the lagoon.
The company behind the scheme warned in October that building work was being delayed by a year to 2017.
Tidal Lagoon Power said it was still negotiating with ministers over an agreed price for the energy generated.
Press reports on Thursday said the company had cut workforce numbers at its base in Gloucestershire.
Swansea council said it would exert all the pressure it could to make sure the project stays on track.
Council leader, Rob Stewart, said he had written to the prime minister to request a "positive and speedy decision".
Labour MP Mr Davies said: "The chancellor specifically mentioned and supported the lagoon in his pre-election budget, and the prime minister came to Swansea during the election campaign on the pretext of supporting the lagoon.
"Now we face this back-pedalling from the government as we fast approach the Paris COP (Conference of Parties) climate change conference where all governments must show willing to engineer a sustainable future.
"It is important that the government gives its full backing to the Swansea tidal lagoon, as the first step for lagoons around Britain and a major export opportunity to help play our part to deliver lower emissions.
"Having said he's for it a year ago, if the chancellor pulls the plug on it, it'll be a disgrace."
Tidal Lagoon Power said in a statement: "Our short-term priority is the construction of the Swansea Bay Tidal Lagoon and the progression of plans at Cardiff and Newport to harness the country's best tidal range resource in the Severn Estuary.
"Our team needs to align with these objectives and regretfully that involves some redundancies.
"Our vision is to provide 8% of the UK's electricity from six tidal lagoons and we have grown quickly so as to take forward these plans.
"However, our first project is now scheduled to start on site one year later than planned and that inevitably impacts the development of the fleet."
A spokesman for the Department for Energy and Climate Change said negotiations on the tidal lagoon were ongoing.
An MP has urged the UK government not to "pull the plug" on plans for a £1bn tidal power project in Swansea.