LONDON (REUTERS) - British Prime Minister Theresa May must seek parliamentary approval for a billion-pound deal with a small Northern Irish party that propped up her government after an inconclusive June 8 election, a legal campaigner quoted government lawyers as saying on Monday (Sept 11).

After losing her majority at the vote, Mrs May's Conservative Party secured the support of the Democratic Unionist Party, which allowed her to form a minority government. Northern Ireland would get 1 billion pounds (S$1.78 billion) extra funding as part of the deal, the government said at the time.

But, citing correspondence with government lawyers, campaigner Gina Miller, who defeated ministers in an earlier high-profile legal case over the process for triggering Britain's exit from the European Union, said the deal could not be executed without parliamentary approval.

Alongside a statement condemning the government for not making the need for parliamentary approval public, Ms Miller released a letter from the government's legal department.

"The agreement does not, and could not, involve the government providing or committing itself to any provision of additional funds to Northern Ireland which would not be authorised under standard procedures, including the consent of Parliament," the letter said.

Neither Mrs May's office nor the legal department were immediately available for comment.

Any parliamentary vote would be expected to pass with the support of Conservative and DUP lawmakers, but could invite further unwelcome scrutiny of a deal which has been heavily criticised by political opponents.

The opposition Labour Party described it as a "bribe" and said the government needed to be clear about how it would be paid for.

In January, Ms Miller successfully challenged the government's plan to begin EU divorce proceedings without first seeking parliamentary approval. After a Supreme Court ruling sided with Ms Miller in that case, Mrs May was forced to pass legislation through Parliament.