MPs have accused the UK Government of adopting a "one-size-fits-all" approach to merging Wales' four police forces.
Home Secretary Charles Clarke wants a single Welsh force
A report from the House of Commons Welsh affairs committee said the government is rushing too quickly towards reform.
Members also said they were unconvinced one Welsh force was the best solution.
Three police authorities have refused to back a merger. Home Secretary Charles Clarke says he will press ahead if they do not agree by Friday.
The government is backing a report which argued that bigger forces, with at least 4,000 officers, are needed in order to tackle terrorism and serious organised crime.
South Wales Police Authority last week joined its colleagues from North Wales Police and Gwent in refusing to support the merger, citing concerns over funding.
A statement from the Home Office said it has been working with the project team to develop their business case and to establish the costs of reorganisation.
It read: "We have recently confirmed that we are able to offer financial support for set up costs to those forces and authorities who are able to proceed now.
"The financial assessments we have made show that there will be savings to the areas as a result of mergers, and we are clear that these benefits will be retained in Wales to be reinvested in policing in Wales."
The committee said the government had given the impression that the merger was a "done deal".
'Gap in policing'
It also accused Welsh Secretary Peter Hain of damaging a consultation process by saying he approved of an all-Wales option before the evidence had been collated.
Mr Hain has said the merger negotiations needed safeguards in respect of local accountability, particularly for North Wales Police where he hoped a deputy chief constable would be appointed.
The UK Government says Welsh forces are currently too small
On Wednesday he said: "It's not a question of the [MPs'] arguing with me, although they have done so, it's a question of them arguing with the inspector of constabulary.
"Despite criticising me for providing leadership on this, and I would have thought one of the things a secretary of state for Wales ought to do and is entitled to do is to do precisely that, provide leadership, they actually acknowledge that there is a gap in policing in Wales."
The committee's Labour chairman, Aberavon MP Hywel Francis, said: "The very short timetable set by the government limited the scope of the debate and impeded consultation with the police forces and authorities.
"The public have not been fully consulted on the proposed changes. We are left feeling that amalgamation of the four police forces in Wales has been foisted on us."
He added that the government appeared to be using Wales as a "guinea pig".
Meanwhile, opposition politicians in Westminster and in Cardiff Bay have warned the merger could mean higher taxes.
A leaked Welsh Assembly Government estimate last week said the cost to council taxpayers in the South Wales Police area could increase by 17% under a single force.
There have also been fears that resources will diverted away from the north and rural areas and given instead to towns and cities in the south.
"We also need assurances that the cost of any reconfigured police service is not going to impact on council taxpayers in Wales, " Dr Francis added.
"The set-up costs must be fully met by the government and any savings made as a result of force amalgamation in Wales must stay in Wales."