The government says a communications mix-up meant it missed the deadline to join an EU scheme to get extra ventilators for the coronavirus crisis.
Ministers were earlier accused of putting Brexit before public health when Downing Street said the UK had decided to pursue its own scheme.
But No 10 now says officials did not get emails inviting the UK to join and it could join future schemes.
Labour is demanding to know why the government had changed its message.
The party's shadow Health Secretary Jonathan Ashworth said: "Given the huge need for PPE, testing capacity and crucial medical equipment including ventilators, people will want to know why on Monday ministers were saying they had 'chosen other routes' over the joint EU procurement initiatives but now they are claiming that they missed the relevant emails.
"We need an urgent explanation from ministers about how they will get crucial supplies to the frontline as a matter of urgency."
He has said the UK "should be co-operating through international schemes to ensure we get these desperately needed pieces of kit".
The EU has said the UK can take part in the procurement project, which will use the EU's buying power to purchase more stock, even though it is no longer a member of the bloc.
But earlier on Thursday, Downing Street said the UK would not be joining the scheme because "we are no longer members of the EU".
The spokesman added: “We are conducting our own work on ventilators and we’ve had a very strong response from business, and we’ve also procured ventilators from the private sector in the UK and from international manufacturers."
Mr Johnson's spokesman denied the decision was motivated by Brexit, adding: "This is an area where we’re making our own efforts.”
The government faced a backlash from opposition MPs following the statement, with Liberal Democrat Layla Moran accusing the prime minister of putting "Brexit over breathing".
Downing Street has now issued a statement saying the UK had missed the deadline for the first round of procurements.
A UK government spokesperson said: "Owing to an initial communication problem, the UK did not receive an invitation in time to join in four joint procurements in response to the coronavirus pandemic.
"As the (European) Commission has confirmed, we are eligible to participate in joint procurements during the transition period, following our departure from the EU earlier this year.
"As those four initial procurement schemes had already gone out to tender we were unable to take part in these, but we will consider participating in future procurement schemes on the basis of public health requirements at the time."
The UK currently has 8,000 ventilators available and has placed orders for another 8,000 from existing manufacturers, but there are concerns about capacity in hospitals as the spread of the virus worsens.
Last week, the government put out a call for other British businesses to convert their factories to make the equipment, and has since signed a contract for 10,000 ventilators with Dyson.
But Boris Johnson's spokesman confirmed the ventilators still needed to go through standards checks and would not be bought and distributed until that happened.
The EU scheme will use the bloc's joint procurement agreement, which helps member states get the medical supplies it needs to tackle cross-border pandemics.
It has also created a stockpile of medical equipment - 90% of it financed by the European Commission - to help EU countries.