US President Barack Obama’s decision to give Congress the right to reject an agreement on Iran’s nuclear programme will create further obstacles for a final deal to be reached by the end of June.

Obama withdrew his opposition to a bill that was unanimously passed by the Senate Foreign Relations Committee, but the president could still veto a potential ‘no’ vote in Congress.

Iran and the P5+1 world powers including the US reached a landmark agreement in Switzerland on 2 April that provides the framework for a long-term accord to limit Tehran’s nuclear development programme in exchange for relief from economic sanctions.

There are already doubts that a comprehensive deal will be reached in June with Iran and the US disagreeing over the timeline of lifting sanctions – the former insisting the barriers must be lifted immediately after the agreement.

The US Republic Party and many of Obama’s Democratic supporters united to demand a role for Congress in the decision making.

The president retains strong powers to veto a potential rejection of the accord and would only require the backing of 34 senators out of 100 to sustain the veto. If the Republicans united against the decision, Obama could afford to lose up to 12 Democratic votes and still proceed with the deal.

“The upcoming Iranian nuclear negotiations present a real opportunity to improve relations for all in the global community, but a finalised agreement by the 30 June deadline is now in doubt,” says Sarosh Zaiwalla, a lawyer specialising in sanctions at London-based Zaiwalla & Co Solicitors

“The interference of the US Congress risks undoing the progress made by the West in Lausanne, complicating an already delicate situation and will undermine the credibility of Western negotiators when they next sit with their Iranian counterparts,” he adds.

UN sanctions and those imposed by the EU could still be lifted if an agreement is reached between Tehran and the P5+1. But a move by Congress to delay the removal of US sanctions could impact Iran’s willingness to agree on the accord.

Iranian Supreme Leader Ali Khamenei recently demanded that sanctions must be lifted immediately after the proposed deal in June.