Opposition to a controversial trade deal being negotiated between the European Union and the United States has intensified amid warnings of its “massive impact” on any future campaign to renationalise the Royal Mail or BT.

The Communication Workers’ Union (CWU) attacked the Transatlantic Trade and Investment Partnership (TTIP) because of a number of “worrying clauses” in the proposed agreement.

The CWU’s annual conference voted to oppose TTIP by campaigning within the trade union and Labour Party movement and to mobilise its 200,000 members.

Delegates at the Bournemouth meeting warned that campaigns to bring the Royal Mail and railways back into public ownership and to halt privatisation of the NHS, will be more difficult if TTIP is approved.

Deputy general secretary Tony Kearns warned that TTIP was a “direct threat” to public services and would give US corporations unprecedented power over economic policy in this country.

“US corporations will be able to sue the UK government in secret courts to force their way into taking over public services, including the NHS. This is nothing more than a tax grab against UK taxpayers.”

Delegates called on Labour to oppose TTIP and go further than its present position of saying the NHS would be excluded.

Opponents have warned that TTIP would lead to public services including health, being privatised, while companies would be allowed to bring claims against the Government at an international tribunal.

The Government has argued that the deal would create new jobs and give a huge boost to business, especially smaller firms in the UK.