Ministers have been told the controversial Chagos Islands agreement cannot proceed for several weeks in another blow to Sir Keir Starmer’s troubled deal with Mauritius.

Conservative peers have warned that legal requirements impose a mandatory delay before any renegotiated treaty can be formally approved by Parliament.

Lord Callanan wrote to Foreign Office minister Baroness Chapman on Sunday, citing provisions under the Constitutional Reform and Governance Act 2010.

Under the legislation, a minimum of 21 sitting days must pass after an amended treaty is laid before Parliament before ratification can take place.

As a result, the earliest possible approval date would be March 7, even if proceedings were to resume immediately.

The agreement to transfer sovereignty of the archipelago to Mauritius while leasing back the Diego Garcia military base was brought to a halt on Friday after ministers were forced to withdraw the legislation.

A Lords debate scheduled for Monday was postponed after Conservatives raised concerns that the deal could breach a 60-year-old defence treaty with Washington.

The 1966 UK-US agreement enshrines British sovereignty over the islands, with neither side having agreed revised terms.

Donald Trump criticised the deal last week, branding it “an act of great stupidity”.

The Government conceded on Saturday that the entire agreement would collapse if the US President refused to amend the existing treaty.

In his letter, seen by The Telegraph, Lord Callanan wrote: “The provisions of Section 20 of the Constitutional Reform and Governance Act 2010 in respect of the ratification of new treaties require that a treaty be laid before Parliament and a period of at least 21 sitting days have expired without either House having resolved in that period that the treaty should not be ratified.”

He asked ministers to confirm they would follow proper procedures for any changes to the 1966 treaty.

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Shadow Foreign Secretary Dame Priti Patel launched a blistering attack on the Government’s handling of the deal.

She said: “Labour continue to mislead and deceive the public and Parliament about their £35billion Chagos surrender.

“They tried to hide the costs and were caught out, and now they’ve been caught out trying to circumvent an international treaty with the US too.

“This surrender by our weak Prime Minister is totally unnecessary, puts our security and defence at risk and hands over our sovereign territory and taxpayers’ money to an ally of China. The Conservatives will continue to lead the fight against it right to the end.”

Downing Street sources insisted on Saturday that the bill would be brought back, claiming the process remained “all in line with international law”.

They accused the Conservatives of “playing parliamentary games” by tabling a fatal motion in the Lords, leaving ministers with little choice but to pause proceedings.

Whitehall officials said British and American officials were “continuing to work together to ensure necessary arrangements are in place for the future operation of the base.”

Baroness Chapman defended the Government’s position, saying: “The Government is committed the joint UK-US base on Diego Garcia, which is vital for our national security.”

She accused opponents of trying to “sabotage the process through parliamentary stunts.”

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