Defence Secretary Geoff Hoon acknowledged on Radio 4: "We know from the US that so far things have not gone as well as hoped for.
A report by an influential cross-party group of MPs says the United States should be encouraged to explore other ways of dealing with potential nuclear threats.
The £41bn National Missile Defence (NMD) scheme is designed to protect the US from missile attacks by so-called "rogue" nuclear states.
But the report by the Foreign Affairs Committee says two out of three tests carried out on the system have not worked and warns it cannot protect against all attacks.
'Destabilising'
The MPs also warn that the opposition from Russia and China to the plan could prove "dangerously destabilising".
The NMD programme works by using radar stations abroad, such as the Fylingdales early warning base in North Yorkshire.
If the programme goes ahead, the UK could find itself playing an important supporting role.
Fylingdales would provide vital tracking data on any incoming missile which would be used to cue other radars, based in the United States, to guide an interceptor missile to the target.
New arms race
Opponents believe the system could spark a new arms race.
The MPs' report recognised that opposing the American system could put a difficult strain on Anglo-American relations.
"A UK refusal to allow upgrading of facilities at Fylingdales would be unprecedented and prove very testing for the alliance," it says.
But it insists the government should voice the very strong concerns that have been expressed about NMD within the UK.
Defence Secretary Geoff Hoon acknowledged on Radio 4: "We know from the US that so far
things have not gone as well as hoped for.
"This programme will continue and further tests are still planned," he said.
Vulnerability
Critics of the system say that the use of bases in the UK will increase Britain's potential vulnerability to attack while not being covered by the US defensive screen.
The committee's report called for an early public statement on the government's position.
But Mr Hoon stopped short of this when in his response to the report.
"Until we actually receive a specific request from the US then there is little purpose in taking a decision that is premature," he said.
Divisions
The Conservatives say ministers are divided over the US missile system.
Shadow foreign secretary Francis Maude said there was a split between the Ministry of Defence and the Foreign Office.
"The MoD appears happy but the Foreign Office contingent of CND, Cold War warriors is coming out of the woodwork, determined to block NMD," he said.
But a Foreign Office spokesman dismissed suggestions of a split.
Both Russia and China oppose the system which they say could destroy the delicate global balance of nuclear weapons.