LONDON (Reuters) - Parliament will block Britain from leaving the European Union without an exit deal, the opposition Labour Party's finance policy chief said on Sunday.

Negotiations are deadlocked between Prime Minister Theresa May's government and the EU on securing a divorce settlement and agreement on future relations, raising the prospect that Britain could walk away from talks without a deal.

But Labour's John McDonnell, leader Jeremy Corbyn's most senior ally, said he believed there was enough support in parliament to block such an eventuality.

"I don't think it's a realistic option, it's not going to happen. I don't think there is a majority in parliament for no deal," McDonnell told the BBC.

Currently, parliament does not have the power to block any decision to walk away. The government has promised to give parliament a vote on whether to accept a negotiated final deal, but that does not provide a means to block 'no deal'.

However, McDonnell said he thought there was enough support in parliament - where May does not have an outright majority - to amend legislation to include such a right.

"They haven't got a majority to get through a no deal situation in parliament. (If) we amend the legislation for parliament to have a meaningful vote, that will force the government to negotiate and come to their senses," he said.

(Reporting by William James; Editing by Mark Potter)