Campaigners for a new EU-US trade deal, which could add an extra £4bn a year to the UK economy, have been given a boost after an undertaking from Europe's lead negotiator that US healthcare companies will not be allowed to run NHS services.

Amid trade union fears that the proposed deal could act as a cover to privatise the NHS, the main European commission negotiator has said that the NHS would be "fully safeguarded".

Ignacio Garcia Bercero, director of the USA and Canada division in the European commission, made the commitment in a letter to the former Labour shadow health secretary John Healey who is chairman of the all-party group on the trade deal. It is known as TTIP – the Transatlantic Trade and Investment Partnership.

Bercero told Healey: "Although health services are in principle within the scope of these agreements and ongoing negotiations, we are confident that the rights of EU member states to manage their health systems according to their various needs can be fully safeguarded."

The intervention by the European commission was welcomed by Healey who is facing a battle in the Labour movement against the EU-US trade deal. The Unite union published research at its recent conference which warned that the proposed partnership places the NHS at "unique risk" in the face of a "Tory drive towards privatisation". There are moves to table a vote at the TUC congress in September and there is pressure for an emergency motion to be tabled at Labour's annual conference.

Healey said the commission letter should answer fears in the Labour movement about the threat to the NHS. Healey told the Guardian: "This letter confirms that the NHS can be fully exempted in any TTIP deal. Now it is for UK ministers and EU negotiators to make sure that the commitments the lead negotiator is making are put in place in full. This letter confirms that the biggest threat to the NHS is the Tories not TTIP."

The former minister spoke out after the European commission moved to allay fears about the impact of the trade deal on the NHS. Bercero wrote that:

• Bilateral agreements made by the EU exclude or contain specific reservations "for publicly funded health services". He wrote: "This policy space means that member states do not need to provide access to their markets for foreign companies and even if they do give access they can discriminate between foreign companies and EU / domestic ones."

• EU member states are entitled to maintain and adopt new measures to control access to their health service market by foreign suppliers. "The EU does not intend to change its approach to health services in trade negotiations for TTIP."

• EU members states face no restrictions on "liberalising the health sector nor from deliberalising it". He wrote: "The EU does not intend to change its approach to public procurement for health services in TTIP."

Bercero also answered critics who warn that the controversial Investor-State Dispute Settlement (ISDS) – the dispute mechanism attacked by critics as secretive – could pose a risk to the "current mix of public and private health service delivery". He said that changes to the NHS over the past two decades have not been affected by EU trade policy.

In an apparent attempt to allay concerns in the Labour party, which plans to repeal Andrew Lansley's health and social care act, the commission official wrote: "If a future UK government, or a public body to which power has been devolved, were to reverse decisions taken under a previous government, for example by discontinuing services provided by a foreign operator, it would be entirely at liberty to do so. However, it would have to respect applicable UK law."

Bercero concluded: "We can already state with confidence that any ISDS provisions in TTIP could have no impact on the UK's sovereign right to make changes to the NHS. I hope that this information clearly demonstrates that there is no reason for fear either for the NHS as it stands today, or for changes to the NHS in future, as a result of TTIP."

The Huffington Post reported Len McCluskey, the Unite general secretary, as telling his conference: "David Cameron ... must use his powers to defend our cherished NHS. US health companies will even have the right to sue a future UK government in secret courts if politicians try to reverse privatisation.

"The most significant effect will be felt in health, enabling US healthcare multinationals and Wall Street investors to sue the UK government in secret courts if it attempts to reverse privatisation. David Cameron can exempt the NHS from these trade negotiations. Unless the prime minister acts, bureaucrats in Brussels and Washington will make the sell-off of our NHS irreversible."

Healey said of the transatlantic deal: "Even on the most measured assessment it could be worth an extra £4bn a year to Britain's GDP."