Boris Johnson is accused of failing to organise his own Whitehall department for Brexit and sending out mixed messages about the role of the Foreign Office in the UK withdrawal negotiations, the foreign affairs select committee has said.

In a report released on Tuesday, the Conservative-controlled committee also said the foreign secretary urgently needed to set out both what he meant by “global Britain”, and detail how the UK wanted to relate with the European Union on both security and foreign policy.

The combined criticisms will add to the sense in both Europe, and within Westminster, that internal divisions and a perceived lack of leadership are making it hard for the UK government to articulate its future relationship with Europe.

Theresa May is due to set out her vision for future defence co-operation with Europe next month in a speech in Munich. The prime minister is likely to offer UK military for EU missions, including permanent military staff at EU military headquarters so long as UK staff are involved in mission planning at outset.

But the foreign affairs select committee also urges her to say explicitly that the UK would like a seat, and voice – if not a vote – on the key weekly meeting of EU ambassadors, the political and security committee (PSC). It meets to agree common foreign policy, often ahead of meetings of EU foreign affairs.

The report suggests UK foreign ministers should meet their EU counterparts monthly – probably before the monthly EU foreign affairs council.

The former foreign secretary William Hague has urged Britain to seek a seat on the PSC, saying there is no substitute for being in the room, but Johnson has held back, saying he does not want to cast the UK in the role of “a demandeur”.

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The Foreign Office did produce position papers on foreign and defence policy last year, but experts interviewed by the select committee found them vague.

An undated detailed paper on the UK’s ambitions for foreign and security policy relations was needed within three months, the committee said, adding it “is important that the government clarify its preferred outcome”.

Similarly, the MPs said the UK needed to articulate its broader calling for a policy statement on global Britain and Europe by July.

The committee added that during its inquiry “we received mixed messages from the foreign secretary, the minister for Europe, and the FCO permanent under secretary. On the one hand, we were told that the FCO was not ‘doing Brexit’ and it was focusing its attention on the capitals of the EU27. However, we were also told that the FCO had a crucial role to play in Brexit and that it was leading on negotiations on foreign and defence policy and on other issues such as Gibraltar and sanctions.”

The MPs added that it was “unclear what further steps the FCO is taking to ensure that its European network can cope with the increased demands of maintaining effective diplomatic relationships with the EU27, without the level of automatic and regular access to the EU27 governments that came with EU membership”.

The committee said the Foreign Office for years had been moving resources away from the EU towards east and west, with the British diplomatic presence in some EU capitals down to two staff.

The MPs conclude: “The Foreign Office is now too thinly stretched in Europe at a time when it has a vital role to play in transmitting information to the UK government, influencing the EU27 and delivering the message that the UK is leaving the EU, but not leaving Europe.”