Campaigners against the terminal believe this will not be the final proposals for the west London airport, especially since British Airways (BA) has already expressed interest in a third runway.
Friends of the Earth (FoE) has long stressed the environmental risk, increased road congestion and air pollution they claim will be created by the extra terminal.
FoE also disputes the claim by the British Airport Authority (BAA) and BA that expansion will create thousands of extra jobs.
Speaking after Mr Byers' announcement in the House of Commons on Tuesday, FoE's aviation campaigner Paul de Zylva said the move mocks the public inquiry into the expansion, which ran for nearly four years.
He said: "Millions of people will soon find their lives blighted by this monstrous new airport. The only winners today are the massive corporations of BAA and BA."
A consortium of 11 local authorities against T5, from across the political spectrum, has warned this expansion should not be underestimated.
Peter Brown, spokesman for the consortium, told BBC News Online: "This is no ordinary terminal, it will almost be the size of the current four terminals put together, and is more like a mini airport."
Nic Ferriday of West London Friends of the Earth stressed this "is terrible news for people and the environment."
"People affected by Heathrow must now prepare for the battles of a third runway, as well as a sixth and even a seventh terminal," he said.
But Ken Howard, of Slough for Terminal 5 and Employment Prospects group, welcomed the decision.
He said: "Last year Heathrow dealt with 62 million passengers and it only has the capacity for 50 million passengers. It was overcrowded and uncomfortable.
"The reason we want extra terminal space is to cope with passengers and give them a comfortable experience at an airport rather than an uncomfortable experience."
But FoE claims traffic congestion it creates will cost £60m a year and the treatment of related health problems it creates will cost an extra £40m.
Any job creation, the organisation argues, will only make up for the huge cuts by airline companies and airports, in the wake of the 11 September terrorist attacks.
"They will also be the most expensive jobs ever, given the estimated total bill for T5 being at least £1.8bn, each of the 6,200 jobs BAA claims will be created will cost £290,000," the FoE's statement adds.
However, Piers Merchant of London Chamber of Commerce, argued it is vital for the expansion to go ahead, to enable Britain to compete economically on the European stage.
He told BBC News 24: "What has to be remembered is you have a 20 to 30-year lead in time for air developments like this.
"So we're looking at 2020-30 before this can fully take shape, so it is vital now if London and the south-east is to compete successfully with Europe."
But Mr Brown said this will not alleviate local authority concerns about the development.
He said: "The environmental concerns of this expansion have been a major factor for the authorities.
"What's got to be realised is that terminal five will have a capacity for 30m more passengers a year, which is currently what the whole of Gatwick deals with."
Job creation
This, he said, will lead to increased traffic on the surrounding roads, more noise from the extra flights created and greater demand for housing.
He said: "We've been told the development will lead to thousands of new jobs.
"But because housing in the area is already very expensive, many of those people employed will not be able to afford to live here, and will have to travel in, creating more congestion on the roads."
Following the go-head for Heathrow's terminal four, Mr Brown disputed expected claims that planning restrictions will curtail the extent of impact on the surrounding area.
"What we were told after terminal four was that planning restrictions would be put in place, but one of those restrictions was that there would only be around 275,000 flights a year out of the airport, but we are now at 465,000."
Construction code
However, BAA has assured local residents that a "Code of Construction Practice" will be put in place before the building of T5 begins.
This will include monitoring from environmental specialists, limits on the impact caused by construction workers, and careful managing of the bio-diversity of the area.
But Nina Bevan, who has lived under Heathrow's flight path for 50 years, said such restrictions would probably mean little.
She told BBC News 24: "Let's face it, we were originally told that there would never be a terminal four, but then we got that.
"After that we were told there would definitely be nothing else - how can we trust anyone anymore?"