Boris Johnson has been accused of heading back towards the “bad old days” of linking foreign aid to the UK’s trade goals, after the government said he wanted all Foreign Office and international development operations abroad to be fully integrated “to promote Global Britain”.

Labour and charities expressed concern about a merger between the departments “by the back door”, as it emerged the government had asked for Department for International Development (DfID) missions in other countries to report directly to Foreign Office ambassadors.

They also raised the alarm about the potential for the government’s new integrated review of foreign policy to examine the current ban on tied aid – the idea that development money is linked to the UK’s trade goals and commercial interests, rather than simply alleviating poverty. This was outlawed under Tony Blair’s government in 2002 with the International Development Act.

DfID insisted that its own ministers would be fully responsible for aid budget decisions “within existing departmental lines”. However, all of its ministers, apart from Anne-Marie Trevelyan, the international development secretary, are now shared jointly with the Foreign Office. Trevelyan has previously expressed sceptical views on foreign aid, once tweeting that “charity begins at home”.

The DfID statement also pointed out that Johnson was “clear he wants all aspects of the government’s international operations to be integrated fully to promote Global Britain”.

Dan Carden, the shadow international development secretary, said Labour was worried that there appeared to be a “clear attempt to merge DfID into the FCO by the back door”.

“Under Johnson’s leadership we are heading back to the days of ‘tied aid’ with UK aid spent in Britain’s commercial or security interests,” he said. “Both departments have important, but distinct, purposes and it must stay that way if the government takes seriously its role in tackling global poverty.”

In response to Labour’s comments, a DfID spokesman said: “It is the UK’s policy to keep aid untied.”

Stephen Doughty, a former shadow foreign minister, said: “It is deeply concerning there now appears to be a back door attempt to divert our development spending to provide sweeteners for trade deals and tying it in a way that would take us back to the bad old days of the 1980s. Not only would this be bad development practice but it could lead to money being spent less effectively and less accountably.”

He added: “It is absolutely clear that there needs to be a cooperative strategy between diplomatic, defence, development and trade agendas but that is a wholly different thing from merging them under the hierarchy of one. This will only lead to skewed priorities and funds not being spent effectively, and may well in fact conflict with laws around the use of our development assistance.”

There were multiple reports before Johnson’s cabinet reshuffle in February that he wanted to formally merge the FCO and DfID but he stopped short of that goal. However, there is still the possibility that this may happen after this year’s spending review, if No 10 feels that the Foreign Office and DfID are not yet working closely enough together with joint ministers.

Johnson launched a major integrated review of foreign policy, defence and development last month, which some view as a precursor to merging the two departments. The prime minister told the FT last year: “If ‘Global Britain’ is going to achieve its full and massive potential then we must bring back DfID to the FCO. We can’t keep spending huge sums of British taxpayers’ money as though we were some independent Scandinavian NGO.”

Charities also expressed concern about the latest developments. Simon Starling, director of policy, advocacy and research at Bond, a UK network for international development charities and NGOs, described the reports that DfID staff were being told to report directly to the Foreign Office as “alarming and suggest the outcome of the integrated security, defence and foreign policy review has been decided before it has even taken place – with zero consultation with humanitarian and development experts”.

Starling added: “UK aid is increasingly at risk of becoming politicised, a move which will make it less effective at reducing global poverty and inequality. The British taxpayer cannot afford for aid to be misspent on vanity projects where there is little evidence that it will reach those who need our support the most.”